How many wounds you have in combat depends on your build though. Most infantry or cavalry units can be relied upon to have 7-10 attacks minimum. That is a 1 at least on each roll. If you are hitting on 4+ then the spell is increasing your hits by 8% or so. That is a fair number of hits.
It's all in relation to the cost of getting the re-roll. You pay for the warrior priest whos stats are not much better than a champion. And the spell might not be succcessful. Just take another demigryph for the same points for a guaranteed 4 extra attacks, and better stats, more unit strength, more wounds, etc...
...i don't want to use demigryphs as a trump to everything in the book so for those 80+ points for warrior priest, you can get:
An engineer
Low level mage or slightly more points for a better mage.
Knights
Archers
Etc...
That spell is terrible. Only time you cast it is if you didn't charge and enemy doesn't have multiwounds. But you don't taje the warrior priest for that spell.
Edit: i was mistaken. Re-roll 1's spell is Signar, not. Ulric. ....so ya, no point in ever taking a sigmar priest.
Edit2: i don't like how some prayers are "command range" while others only affect the unit he has joined. Needless complexity. Such a terribly written book.