Mages *will* cast relief after turn 5. I don't know what conditions encourage it but I have seen relief casters cast it repeatedly through long battles, much to my relief, so to speak.
True, you can't count on it...but then that applies to most mage scripting
I don't know if you are already doing it but I tend to make at least some of my communion masters into communion slaves also. This does take up one of their 5 guaranteed spell slots so it's more useful in long battles (then again, I don't bother with communions early on or in small battles unless I *really* need a specific spell).
It may also be what you are casting and who your masters are since, as you knwo, the more paths you have, the cheaper each spell is.
- Kel