I am no pro on this or anything, but from the Doms I play, I'd have to say it totally depends on two things, 1) your play style and 2) Which powers sets you have. ON my Fire/Fire Dom the AoE Immob. comes in handy when used with things like Flashfire, also keeps them nice and in line for Fire breath! But on my Ice/Ice Dom it feels like a death trap to even use it. First off it totalyl kills the powers like Ice Slick, so I go from guys I could totally take out that are falling on the butts to guys who started off falling now are rooted and not too happy about it!
I also happen to have two earth Doms one earth/thorns and the other earth/fire. for the one with Thorns I don't have my AoE Immob at all, but do for my Fire one, so like I said it's kind of depends on your play style too. I am sure one of the players that know way more about Doms then I do will give you some great advice on the pros and cons of the AoE immob. 
IMO: solo, it's OK because everything is attacking you anyway. Teamed, use it at your own risk. It's a real suicide button in the early levels. I imagine more than a few players have quit Doms due to constantly dying in those early levels after throwing out AOE Immobilizes. Once you're an experienced Dom player who knows what your're doing, AOEIs can be leveraged in effective and interesting ways. But as a general rule, especially for those new to Doms, skip them.
P.S. Single target Immobilizes are better in Archvillain fights. And in PvP.
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Pretty much.
Find out whether the AoE immobilize is going to be working with you or against you based on your playstayle and power choices. (you didn't mention what AoE you were referring to)
Just remember that unless they are slotted for a lot of range, they should never be used as an alpha attack. .. and that targetting one while out of LoS with up to 15 others is as good as an AoE hold for you.... except for that one ( like from around a corner ) It becomes a pace setter.
"Situational power? Sure. Although in a sense... all powers are situational. It's just that some situations occur more than others." Understand the situation needed in order for the power to be most effective... and make that situation happen.
Yes, I'd take it; but you have to learn when to use it without drawing too much agro.
Spines/ D A lvl 50 Scrap, stone/wm lvl 50 tank, Kat/reg lvl 50 Scrap
Grav/Kin lvl 50 Cont, Fire/Enegry lvl 50 Blast
Warshade lvl 50, PB lvl 39, nightwidow lvl 50, crab lvl 42
plant/thorns lvl 50 dom, ice/fire lvl 40 dom, grav/nrg lvl 41 dom
Generic rule of thumb: If you have an AoE stun, following up with AoE immobilize is very, very good.
If you don't have an AoE stun, a group immobilze can still be useful. But never open with it as a primary attack.
My level 50 Dominators:
Madame Mindbender 50 Mind/Energy
Fly Agaric 50 Plant/Thorn
Nate Nitro 50 Fire/Psi
ok ill prolly take them . im lving a fire/psi, and a grav/nrg and i and wanting to be more useful on teams.
Just got my first dom to level 50 last night! Earth/Elec, and I didn't pick up my AoE Imob until I had Stalagmites and it was slotted some. It works nice for a PvE build pared with Stalagmites and Soul Drain from GW. It's also not bad with Volcanic Gases or if you team with a /Storm, Ice, or anything else with a scatter rain. Not to mention if you are on a team with more than one Dom they will most likely use it when you drop Earthquake anyways lawl. Happens to me ALL the time.
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Schatten-Schlag (LvL 50 Nin/Nin Stalker)
Crocell (LvL 50 Thug/Poision MM)
Victor Smart (LvL 50 Rad/Psy Def)
The Masked Mayhem (LvL 45 Fire/Fire Blaster)
For a farming dom, always take the AOE immob. Other than that, it is situational. For example; earth, the AOE immob not only prevents knockback, but also is a 1.17s cast time ranged -def. That might be useful given your opponent.
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I didn't on my ice/psy - of course, Ice Slick does wonders for keeping things in an area, and Arctic Air keeps enemies from running away too quickly.
Plant/Thorn, I can't imagine not having it - just to keep a group together for the other AOEs pre-Creepers. It was put off a little bit, though.
should i take the Aoe immobs or skip them? whats sets are they useful in? i keep hearing different things.