from what I've seen, mage heavy rooms in perilous fall apart on the boss unless they are skilled or have a good AW who can solo it. it's the one place where aoe abuse doesn't work so well.
it's usually if one falls, the rest do too, especially if they were relying on a rogue for all their dps and the rogue goes down from having to carry too much.
A bit of housekeeping first:
1. You say "skilled", but what you really mean is "competent at casting Barrier".
2. Fixed your AW comment.
3. With competent support, there is no reason why a DPS would go down from "having to carry too much". If you have a reliable Barrier coming all the time, the only reason you go down is if you overextend and get out of range of the Barrier caster. That, or lag. Accursed lag. The ancient Tevinter magisters entered the Golden City and saw that it was laggy. And they despaired. Then they were cast back into the mortal realm filled with mindless rage, bringing a blight upon the world.
Your observation AoE "doesn't work so well" is true only with regards to the boss. Of all the mechanics in the game, single target spike damage is not a mage forte. That said, the constant swarm of mobs through a doorway (how most people tend to play) makes AoEs extremely useful. Things like Fire Wall, the Mine spells, rogue mines, Whirlwind, etc all shine in doorways. Against the bosses, having those Barriers means you can afford to semi-kite them around, whittling them down with the AoEs meant for trash, and just finish them of towards the end. It's not like zone 5 ends the moment the boss dies, so the order in which you kill doesn't really matter, provided you can survive, of course.