adding relief casters generally does nothing, other than giving you a fallback in case one is killed. the problem is that after the 5 scripted spells, your relief caster will often decide to cast whatever useles bit of drivel wanders into his head
so long battles, especially any serious castle assaults, will usually end up w/ all your communicants dead. having a communion master casting earthpower or some such will certainly help, though earth mages are fairly rare for the pyth.