I personally use Rain of Fire as my "oh shizzle !" emergency control power. When I know I'm being overwhelmed, a RoF gives me some breathing room.
Advantages: Mobs scatter and stop attacking you; the large area of effect ensures that most (if not all) of them will be hit. You now have time to fly down to refresh your ice patch; to use a few inspirations; and to recharge your critical damage powers.
Disadvantages: This will aggro every mob. Onto yourself. Once the fear effect wears off, they will all hit you at once, killing you instantly. The obvious solution is to make sure that, by the time the RoF expires, nothing is left alive (er, I mean, un-arrested) to damage you. I can live with that.
So, yes, it's a situational power, but a worthwhile one, IMO.
