"You are confusing NPCs in the tabletop sense with party NPC companions (which are in fact not truly NPCs since you play them). Tabletop NPCs don't even have classes."

Read to the end of Chapter 4 in the DMG where they provide some extra NPC villainous class options. The Castellan in Keep on the Borderlands is an NPC, and he had a class. Narlen Darkwalk is perhaps the greatest NPC of the whole BG series. I am not saying to *never* tie the persona to the class, I am saying that it might be unique and fun to try maybe *one* NPC with a player's choice.