As well as people debuffed with surrender to fire.
I also bvelive that it raises your's resistance against fire, so less damage taken from fireballs and such flying your's way.
Not only are there plenty of fire weak mobs - as pointed out, it's possible to cast mass surrender to fire in a pole group (if you have a warlock there for Queen).
30% extra damage are, of course, something to completely scoff at - I mean, who'd want it?
Mmkay, since I havent seen any increase in damage when soloing or in party normally. But then again, noone is casting surrender to fire on every **** mob.
It converts your physical damage to fire damage, try hitting something with fire resistance/weakness you'll see difference there. Watch out using it in pvp though, if your target has resist fire and elemental protection on, infernal form will reduce your damage *alot*.