[Gimli]

One big plus to switching a Bishop to a pure class is the +25% bonus to that school he'll get, will help you cast spell in that realm at a higher power level more safely. A 125 in a school is alot better than 100. It all depends on the party makeup too; I've had parties where my bishop was the lone pure caster, there I liked to have him learn all 4 books and stay a bishop. Had one party with 2 bishops, both 2 book guys, and a pure priest. One bishop I had as a priest/alchemist and I switched him to pure alchemist because I already had plenty of priest spells; the other was a psionic/mage, switched him to a pure mage because I think that school has better spells, plus I had a monk for more psionic magic and no other mage spell caster. That was in expert mode and it worked beautifully, you really want to get your spells cast at level 7 there as much as possible. I switched at level 18 because you can access level 7 spells there, but that also depends on how far you intend to level before you finish the game; some people like to go really high before they finish it, I don't reallly go for more than say one extra level once I'm done with evertyhing I need to do before the end of the game.