My opinion has always been that IOs are a lot more useful solo than on a team. Support ATs are so ridiculously overpowered in a team environment that they completely overshadow the effects of IOs. I could spend several billion softcapping a toon OR I could use SOs and find a FF defender (or a pair of Cold or Traps Defenders). Once you stack 3 or 4 support toons on a team you pretty much break the game. In the case of the higher tier sets (such as Dark) you don't even need that many.
That being said I do think cheapish IO builds (a mix of Frankenslotting and some cheaper sets) do give a very good return on investment and make things that much easier. In the case of dark Frankenslotting means you can afford to add more -to hit slotting in without needing to sacrifice damage/accuracy/recharge which helps push the team to higher difficult by countering the purple patch.
I noticed that my expensive build toons don't really shine as much in a team environment as they do solo as well. Take my Ill/rad, for example. I spent a stupid amount of inf on him to get Perma PA as well as a few other effects that I really wanted and he can handle a GM with ease. But put him on a team with defenders or corruptors and all of a sudden I'm the guy tossing down controls, debuffing with rad, helping with damage etc. Just another member of the team. No more helpful than the SO'd FF defender or the SO'd blaster on the team. This isn't a gripe. It's actually a really good feeling. On teams it doesnt matter nearly as much how uber you are and I start to question why I even cared so much about perma to begin with. I am very grateful for my D3. It reminded me you dont have to do obsessive number crunching in order to enjoy the game. You just need a fun concept and 7 other sleep deprived nerds at 3am.
Yep, that's the reason I keep returning to this game despite periodicly trying other games (MMO or otherwise). The teaming dynamics are what really make the game. I still have several heavily IO'd toons but I do it because I enjoy having heavily IO'd toons not because I need them.
I pimped out my main, and usually only throw end procs (miracle/numina/perf shifter) on end heavy toons, or sets for key 1-2 powers just to get more bang out of them. I like the advancing in lvls and trying new powers, or altitis, too much to spend all the time it takes IO'n a lot of toons. A full team of 8 ppl with the right mix will plow through stuff. I'd be hard to have 8 of anything and not be effective really.
You can get some benefit out of IOing your defender, but you really need a purpose for it. For my defender, I wanted to not die. Ever.
So I basically IO'd defenses and resists, and it worked: I can actually take alphas on pulls and walk away with barely a scratch. It's been a wonderful game-changer. As a defender, the weakest link in the team always felt like it was me - My debuffs were keeping everyone else alive just fine, but if the enemies started targetting me, and I dropped, then things might start falling apart. That fear is completely gone.
It just opens up a ton of new and ridiculous things you can do. Tankfender is awesome.
There are other things you can do with an IO build, it's all down to what you want to do. If you're perfectly happy with your defender and see no way to really improve your style of play, then by all means, don't IO! 
Ghostveil, as drawn by Shia
I like my D3, I really do. However it's a "tier 2" character for me, meaning it's not the first one I think about for whatever's going on.
I mention that because that means I decided to give him a good, but not great IO build. That is, no purples or anything terribly expensive. Did it make a difference to go to the "good" IO build from the non IOd build? Absolutely. I went for building up a decent amount of recharge, mostly to be able to use Howling Twilight on tough targets more often.
So no, it's not overkill to build up bonuses for a D3, it just gets better.
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I got a D3 slotted up for damage and alt build for debuff with set's in them both. I personally like it for my play style and ether way i'm good solo or team friendly But i feel sets give a bonus that the IO's don't give.
Not a fan of grammar, spellcheck or proper sentences.But will have fun anyway
I'd say yes, a D3 is just fine as is. But I still IO'd out mine - personally I went for +rech (more Tar Patch, Howling Twilight, Fearsome Stare, and a smaller gap in the Tentacles-Nightfall combo, etc), and +dam / procs. I'd agree that I see the most improvement from this in solo situations, but the more frequent -res and extra damage helps in teams too, especially in AV fights.
So I've been working my dark/dark/dark defender up and he is sitting at 42 while I am distracted by the new i19 content. But I've been thinking about it while planning his IO build in mids and I'm starting to think that any IO sets put into him would kinda be overkill. I know, there's no such thing as overkill. But really, every time I get on a team with my D3, its like we just went into easy mode. Tossing out so much to hit debuff that we breeze through the content. My friends have been upping the difficulty when I use him but still we aren't getting any real issues. Now I'm sure I could dump cash into him to make him even better, but at the moment, I'm just using SOs and he has no endurance issues, does his job of debuffing very well, heals respectably, and I never feel like I'm waiting for some all important power to recharge since there are so many options available to do the job. I guess what I'm asking is, is this just D3s that are like this? Have I been dumping inf into min/max IO builds for so long that I forgot you can do a great job with just SOs or common IOs? I'm really starting to rethink the necessity to get expensive IO builds for all of my toons.