I can see cases where evil NPC's might want to stay in an otherwise good-aligned party. If the evil or neutral NPC is motivated strongly by greed, then maybe from time to time certain amounts of gold, or maybe some magic items, might mysteriously go missing from the team inventory, heh heh. Or if the evil NPC is out for revenge against some other story character, perhaps when that other character is finally encountered, the evil NPC drops out of the party to attack. There are lots of situations where evil (or good) NPC's might suddenly decide to do their own thing for a while, and then maybe even come back to the party. Edwin does this a little bit in BG1 ... if you don't kill Dynaheir he gets mad and leaves, but then later you run into him and he offers to rejoin. And don't get me started on Narlen Darkwalk, the greatest independent NPC of them all!
I like that kind of complexity in the NPC behaviors because it makes them more real, and it lets the player know that not everything in the game revolves around the protagonist.
