We've always played it differently. The rules aren't explicit, but they do say you use power to boost spell cards. Not just spell cards that require power.
ASFIK you can add power to any spell. If it is a duration based spell this extends the life of the spell and if it's an instant it adds to the damage or effect of the spell.
You're both right and wrong. You can boost temporary spells to extend their duration, even if they don't mention power explicitly. Also, while i think nothing prevents you from boosting fireball, the effect is always a flat 5 damage, so the boost would have no effect. Only if the spell card said something like "2 plus power of this card" would it make a difference. So yes, you can boost any spell, but in some cases that won't make any difference.
EDIT: and like the previous poster said, boosting spells replaces the original power, it does not add to it.