RChu1982: I believe that it's a non-issue for level 6 and under spells (having maxed skills). With level 7 spells, the cheaper ones (Tsunami, Cerebral Hemorrhage, Concussion), can be cast at power level 7. It's the expensive ones: Restoration, along with the hit-all, and kill-all spells, that can only be cast at power level 6 safely.
But that's with primary skill or item bonuses (and most of the items in question are dropped by very specific enemies that aren't that common).
RChu1982: Bishops should give up more, you say. They get the same chance to cast level 7 spells at power level 7 as the specialist casters, while having way more magical options. However, they suffer severe penalties upon leveling up, so they will always trail behind the rest of the party in levels. I guess it evens out?
Actually, the chance isn't as high; at max skill a Priest is still less likely to fail PL7 Restoration than a Bishop.
The thing with Bishops is that, ignoring spellcasting, they end up comparing a bit too favorably:
* Priests get 25% more HP and the Pray for Miracle ability that nobody uses. Aside from that, their equipment selection and fighting ability isn't any better than the Bishops.
* Alchemists get *slightly* more HP and can make potions while resting (nice, but minor), but are more limited in weapon selection (no mace/flail, so no good options that allow shield use).
* Psionics have terrible equipment and worse HP; the only thing they get is mental immunity which, while useful, is still situational. (And it doesn't protect against a certain artifact.)
* Mages are even worse, with Psionic equipment and even worse HP. All they get in return? A measly 5% resistance bonus.
Incidentally, I have been thinking about what I would change if I were making Wizardry 9, and one idea I've had is to require that the character be at least level 5 before changing into a hybrid or bishop/ninja. This would make those classes not for starting characters, and would, IMO, work better with multiclassing (which I'd try to make behave more sensibly). This, incidentally, means that a Bishop can't be optimal with more than one spellbook. (On the other hand, some other changes would make weaker offensive spells, like those from less favored spellbooks, more viable.) (Also, if a character can't become a hybrid until level 5, to compensate there'd be no level penalty to their spellcasting, and there's the option of whether you want to go from a fighter, a full caster, or a mix of both.)