They have long recharges and don't last nearly as long as the single-target holds. In the time it takes for the AoE hold to recharge, you could hold each mob in a group of enemies with your single-target hold. AoE holds - to me - are pretty situational. Also, the enemy doesnt usually survive long enough for the effect to be of any use.
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Depends on the Set, to be honest.
Bless my Ice domi, she has no psuedo hold like fire (Immob & Stun), so the mass hold is pretty essential if you want to get some hard control in on the team...otherwise it's just ice patch, slows, popping lucks like they're going out of fashion.
..that or mass sleep..because *so* many teams work well with sleep powers, right?
I find it's useless on my fort, and mind/ sets, as cone fear, mass sleep, mass fuze = up faster, just as good.
All in all, as I say, depends. My domi's recharge is stupid high, so I'm using glacier almost every second mob, and few other powers make me smile as much. However, i hate hate hate flash on my illu/ troller, as the recharge, base acc, and everything about it is dumb.
Can't offer any sweeping generalisations i'm afraid 
Except that on a Fire/ take if it you can fit it in, certainly helps to have it if your ghetto-hold doesn't do the trick
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They have long recharges and don't last nearly as long as the single-target holds.
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Initially
Domination, Power Boost, Megalomaniac, Glacier....
*goes downstairs to make a coffee while Jack does the rest*
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I've heard a lot of bad things about AoE Hold powers... except the reason. Why are they so seemingly unpopular?
I just picked up Cinders on my Fire/Fire Dominator. Aside from missing like crazy because I'm on DOs and it has 0.8 base accuracy, it seems just fine. Quick activation time, and I can use Combustion in safety after using it.
Is there something I'm missing and should I respec out of it?
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Dominator Mass controls used to have a target cap of 10 until relatively recently, whereas Controller mass controls had a target cap of 16.
It's not much, but on a full team, only being able to lock down part of a spawn rather than most of a spawn can be significant.
And solo, Mass holds aren't really too useful, due to the silly recharge times they've had for a long while now. IME, they're only available every third to 5th spawn, so you're spending most of your time coping without them available.
Both of these may have contributed to the general opinion of mass holds, but I can't say for certain.
Warning:
The above post may contain Cynicism, sarcasm and/or pessimism. If you object to the quantities contained, then tough.
A lot of this stems from pre-IO days. Even with 6 SOs they aren't the I Win button they used to be (it used to be silly, nearly every mob held straight away).
With IOs though you can get decent Acc and maxed out Hold and Recharge on them now, well worth it in my opinion.
Some of them have extra restictions which make them a bit of a pain. E.g. Glacier is pbaoe, and requires you to be on the ground. ranged aoe holds are much more convenient.
I really should do something about this signature.
I don't know what some people have told you but on my doms I couldn't live without my aoe holds. Although I am a dom fanatic 
Once slotted up with some IOs and maybe a couple of HOs when the cash is drummed up, you can use your hold very often. Remember that you can alternate your aoe controls, on my gravity I use wormhole as my main aoe control and use my aoe hold when it isn't up or when its more use - aggroing 2 mobs, ok worm 1, aoe hold the 2nd - room stops doing much
Plus, global rech and hasten make a huge difference. imo take the aoe hold and you'll be happy once slotted up more.
... more convenient yes , but PBAoE are a very good way of escape in case of emergency (mobs re-spawn, tactical errors...)... I play a ice/psy dom, and glacier have often save my life . moreover, With this primary PBAoE are great because of artic air power which generate fear & confusion on the escape mobs . So, due to my secondary (psionic assault), glacier greatly help drain psyche power caption .
Morevover (one more time) , a dominator is designed to entering melee during fight, and PBAoE have greater radius than cone range .
So, really depending of the primary/secondary and of the playing style, both have their fonction and application in a domi build ihmo .
Well, I hang out in melee anyway as it is. So the PBAoE versus ranged AoE thing doesn't seem to matter much.
So it's more of a matter of opinion rather than a hard fact that AoE Holds are so-so?
Glacier is one of my fave powers in the game and has a fantastic sound effect to go with it. Put then as others, I've spent silly amount of my Domi, and his aoe hold has a purple set in it, and is up about every other mob. Nothing quite like beating a brute to the mob, hitting glacier and then telling the Brute to find his own mob
They are not that great at the lower levels, and are very situational, but then, is any domi good at low levels?
Take it and use it as you need it, not just cos it's up.
I've heard a lot of bad things about AoE Hold powers... except the reason. Why are they so seemingly unpopular?
I just picked up Cinders on my Fire/Fire Dominator. Aside from missing like crazy because I'm on DOs and it has 0.8 base accuracy, it seems just fine. Quick activation time, and I can use Combustion in safety after using it.
Is there something I'm missing and should I respec out of it?