I find that Mages may seem weak at the beginnig, but later on, while the most damaging Priest spell is two Cloud of Blades, while both of my mages are pumping off 8 Fireblasts.
I find Priests valuable for their healing/Curing And my Mages valuable for their damage spells.
I never summon/charm, and only just before big fights (Vahkos, Nolagh-Kar, etc.) do I use hasting/Blessing/Shielding.