Well, SOME people have 8th level spells at this point. Others just have +15 BAB and some damage dealing abilities. The ability lets you burn down everything in sight, which could be quite a lot due to the lack of any range limitation. It's not just mook killing... because you bypass hardness, you can burn a hole straight down to a mindflayer city and incinerate the entire thing, all while staying 5+ miles above ground where they can't hope to see you.
JaronK
I'm not saying you
couldn't. I'm just saying that by the time you hit 15th level, the CR system would have you believe you should be capable of quite a lot. I guess an
army of CR 7 creatures is an exaggeration, but you're still expected to be able to take on 16 of them in a group of 4 PCs and only use 20% of your resources. 64 might be considered a "boss" fight. So, that's not taking any specific
class into account, but just levels and CR as printed. Of course, some classes can do this magnificently, and others fail pathetically.
My point was, by the time you hit 15th level, the game has changed
substantially. Sure, having a fire mage running around in a world is dangerous. If he decided to wipe out armies and towns, he could totally do that without breaking a sweat. Of course, a 15th level caster could do the same. Hell, given enough
time, a 15th level
fighter could do that.
In an actual fight against a 15th level opponent, Rain of Fire isn't that scary. You're doing 52.5 + Cha mod damage per round.
What Rain of Fire does that some other PCs don't is it ruins
plots faster than what some of the other PCs can.