[Nerull]

Actually, when I use a specialist wizard, I usually will pick up another wizard to cast the missing spells. For example, I always end up using Xzar and Edwin together. Then, I concentrate on those areas where they two differ. For example, illusion has Mirror Image, the two invisibility spells, etc. which are more defensive in nature. Thus, since Xzar can't use them, I make sure to give him the better defensive items (a certain claw comes to mind, along with a certain belt and a certain pair of boots) of the two wizards and focus his spellcasting on more destructive areas (that fits a necromancer, anyway) and for identifying items. I then focus Edwin on things like Invisibility, etc. and focus him on destructive stuff after that. Thus, they complement each other.

Of course, another tactic is just to say "I don't have access to those spells, so work around it". In BG1, that is relatively easy. I usually play all-good parties, so end up using Dynaheir as my only mage. Can't cast Conjurations or Enchantments. Ok, go get wands to supplement. Especially easy with Dynaheir and Xan, but harder to do with Edwin (not really needed, just spend the 100gp to identify stuff) and Xzar. Even without wands, you can just work around those things, especially if you are "tank-heavy" in your tactics.