Here is some info you may want.
If you specialize in:
Abjuration- cannot cast alteration spells
Alteration- cannot cast Abjoration or Necromancy spells
Conjuration/Summoning- cannot cast Invokation or Divination spells
Enchantment/Charm- cannot cast Invokation spells
Divination- cannot cast Enchantment/Charm or Conjuration/Summoning spells
Necromancy- cannot cast Illusion spells
Illusion- cannot cast Necromancy spells
Dynaheir is an Invoker so missed out on many spells, Xan is an Enchanter, Xzar is a Necromancer, and Edwin is a Conjurer.
So before adding an NPC to the party, consider what spells they will not learn. Specialists in general are better since an added spell per level is desirable at higher spell levels.