Inner fire is a self buff for the priest, not a raid wide buff. Priests are balanced around having it active.
Quoting for emphasis.... There is no debate on this one. When people talk about spellpower buff they mean things like Arcane Brilliance. For a spriests there might be fights where it might be benificial to temporarily switch to Inner Will (I can't think of any off the top of my head but, you know, it's possible) but the majority of the time you always want Inner Fire to be active.
Now I just need someone to post saying we need to stack melee hit so fiend won't miss and my day will be complete