In short, yes, there are times you would want an exemp/PuG build that you wouldn't ordinarily take on your regular leveling build. Fortunately, due to a recent perk introduced by the devs, you can create an alternate build starting at level 10! Usually on my alternate builds, I don't sink all kinds of money in crazy expensive enhancements, just generic IOs and single origin enhancements. You can change which build you are using every 15 minutes or about the time it takes to do a mission.
Triumphant Defenders Forever
Psylenz FF/Psi, ArticQuark Storm/Rad, Symon BarSisyphus Bots/psn, Max VanSydow Thugs/Dk, Cyclone Symon Bots/stm, Blue Loki Ice/Cd, Widow 46526
HelinCarnate:OMG it is so terrible. I have the option to take 3 more powers but no additional slots. Boo F'ing hoo.
One of the nice features of a dark/ defender, much like a rad/, is that virtually every power you want for a team benefits you solo so you often don't have to make different builds for regular play, unlike an Emp or FF who might want something different for soloing vs teaming. Even Howling Twilight is a nice AoE stun.
And you are right about defenders often exemplaring down, for lower level TFs and such. My suggestion there on your final build is take relatively few attacks and more "major" defendery powers compared to your leveling build. Its a safe assumption that if you are being asked to support a team for a low level mission, you aren't generally being asked because of your l33t damage. 
By definition, if you are exemplared you are teamed, unless you are doing Oroborous for something, and if you want to solo a lot of Oroborous, make a good soloing alt like a scrapper for better speed. Plus, on a final build, you are likely to have your handful of lower level attacks slotted with some recharge, or have some global recharge from IOs, so your few attacks make a better attack chain then when you are actually that level. You may even have a vet attack or two by then, which are great on a defender, they are some of the best damage you will see, since they aren't pegged to the low AT modifier. I do suggest at least one AoE, if not both, since again exemping down means teams which means lots of foes.
You could even consider dropping a travel power on a final build or taking it late, relying on Ninja Run (if you buy the booster pack) and sprint. With a stealth IO and your stealth aura, you could ghost missions, which is handy for lots of TFs. That argues for fitting Recall Friend in early if you can manage it.
Unless you're going to be only doing shard, LGTF, ITF, STF, and Kahn, exemplaring has quite a bit to do with your usual TF regiment. As such, be mindful of what are your all star power, what you find usually more useful than others, and focus on what your sets shine in. I typically, unless teaming with a friend, only exemplar down. Before Ninja Run, I didn't take my travel power until mid 20s. Now it's iffy if I take it at all or late (30s or right before 50).
50s: Bla- Arch/Mental Cont- Mind/FF, Earth/Cold, Ill/Therm, Earth/Rad Dominator- Plant/Psi, Elec/Earth Corr- Fire/Storm, Arch/Sonic, Rad/Kin, Beam/Sonic, Psi/Time Stalker- Elec/SR Def- Storm/Dark, Emp/Psi, Dark/Elec, FF/Arch, TA/Ice, TA/Elec, Kin/AR, Cold/DP, Traps/Psi Scrap- Fire/Shield Tanker- Dark/Mace, Ice/Kin Brute- Claws/WP, SS/Energy, BS/Elec
This is something I've given a fair amount of thought to. A couple of things to keep in mind.
1.) You'll have access to all primary, secondary, and pool powers up to *5* levels over your exemplared power, as well as all temp powers.
2.) You'll have the benefits of all enhancements slotted . . . but depending on your level and how far you are exemped down, the enhancement value will be scaled down according to a formula http://paragonwiki.com/wiki/Exemplar...n_Enhancements . You can do some funny things with frankenslotting set IOs to minimize this effect. The big thing is that enhancement values under +20% don't get scaled down so long as you are exemplared at level 21 or above. You can use +x/+y set IOs level 25 or less, or +x/+y/+z set IOs level 43 or less and get full value for the enhancement.
3.) You'll have set bonuses to the extent that you are within 3 levels of the lowest level of the set. For example, if you have 2 level 33 set IOs, 1 level 35, and 1 level 36 of the same set, you'll have the 4 slot bonus at level 33, the 3 slot bonus at level 32, and the 2 slot bonus at level 30. This isn't directly related to the level you choose you power at, because while exemplared you have access to set bonuses even if you don't have access to the power . . . so long as the IOs are w/in the level range. And you can do some neat tricks with this, too.
4.) Generally you'll only be exemplared down for TFs or maybe PVP. So there are cutpoints where you can worry about whether or not you'll be effective at that level. If you're just going to help someone with a random mission, its probably going to be okay either way.
I think you've hit the nail on the head when it comes to travel powers. Flight isn't the fastest travel power in the first place, though it is very convineient. If you are prepared to hoof it a bit (or buy a jetpack), you can squeeze two extra powers in.
Dark Miasma is pretty straightforward. So many its awesome powers are must-haves. They really shine. You'd want to take them anyway. Dark Blast is a little different, so here are some thoughts on its powers with exemping in mind.
In general while leveling part of my calculus in choosing powers is "can I spare enough slots for this power to make it shine?" A power might be awesome when slotted properly, but if its sandwitched between 2 other *more* awesome powers the logical thing to do is put off taking the power till you have the slots to make it shine and pick up something that would be useful straight out the box. Dark Blast has 4 powers that (IMO) deserve a second look in an exemplared build.
1.) Tenebrous Tentacles: I personally love this power and take it when I can get it. But it might get lost in the shuffle, especially if you are trying to slot up other powers. For me, its the little power that can. It debuffs acc, it immobilizes, and its an aoe power. What's not to like? You can slot it up with 4 different sets and there are a variety of interesting procs available for TT. This might blow your mind, but the purple immobilize set has a chance to hold and the purple TAOE set has a chance to knockdown. Debilitative Action is a basic IO set, and has a chance to stun. There are a variety of damage procs, plus some others that you might want to consider. You can turn TT into an "I win" button.
2.) Night Fall: combined with TT, you can pump out some pretty decent AOE damage once you get it slotted up. While leveling you might want to put it off, especially if you team a lot. But in an exemplar build, you want this power ASAP.
3.) Gloom: you've probably thought of Gloom already, but just in case you haven't its a heavy hitter. Throw some tri-aspect set IOs in it, and you're cooking with grease! You might be tempted to leave it out of an exemplared build, thinking that you aren't on the team to do damage, but goldarnit . . . its *really* good.
4.) Dark Pit: pretty much worthless unless slotted. And not *especially* great even when slotted. But I've found it useful while exemplared. Its one more soft control.
Some situational pool powers to consider:
Recall Friend: you have shadow's fall, right? Put a stealth IO in sprint, you're invisible! Not bad if that's what you want to do.
Hasten: for AV fights, bring Howling Twilight back up that much sooner. -Regen powers rule AV fights, and that's a big part of what you'll be facing while exemplared. Note that the Positron TFs don't feature an AV.
With that said, I'm assuming that Twilight Grasp, Tar Patch, Darkest Night, Howling Twilight, Shadows Fall, Fearsome Gaze, and Dark Servant are going to be in your build regardless. I can't think of any reason *not* to have any of those powers within a level or two of when they were available. And you might want to think long and hard before attempting to go without stamina.
On reflection, you might want to take a strong look at the leadership pool, also. You'll be bringing some noticible + def to the table with Shadows Fall. Why not make it better with Manuevers? At lower levels, the extra + def will go a long way as compared to at higher levels when folks will have more of their own defenses fleshed out. And if that makes sense to you, then consider also assault and/or tactics (especially if you want to focus more on support and less on blasting).
A lot of it depends on playstyle. I'm more of an offender, so I dig having some solid blasts on hand. Even if you're more inclined towards support, having some potent offensive tools in your kit can be useful. Don't neglect TT! :-)
So, it seems to me that one of the big attractions of Defender is that you can just about always help people out. Whatever they want to do, a Defender makes it easier.
Which means you might spend a fair bit of time below your current level.
Are there cases in which the order you'd pick your powers in while levelling would be noticably different from the order you'd pick them in if you were freshly speccing in order to exemplar down well at a broad range of levels? If it matters, my build is dark/dark, and I'm only level 23, so this is only very marginally relevant. But, for instance, at 14, I took flight, because being able to fly is a huge win for me both in convenience and for RP purposes. But if I were in a team, I might not care whether I had flight; I might prefer some other power that I can use more within a mission. (Flight is a bit of an endurance hog to keep on during a mission.)
Thoughts?