Xan isn't turrable, but he is more of a crowd controller than a damage dealer. He is also one of the potential companions where, if you turn them down (and leave the area?) they will disappear from the game, but if you hire him, you can immediately kick them and they will stay where you left them.
With him you toss out sleeps and charms to take big problems out of fight for a few rounds, while you kill the rest of the mob. CC is really strong in BG, and a good sleep or charm can drastically change the difficulty of some fights. The trade off is, he cant cast the best damage spells in Magic Missile or Fireball.
He still has access to Melf's Acid Arrow, Flame Arrow, and Skull Trap; which aren't that far behind the Evocation spells, if you want him to nuke things. If you keep Dynaheir, she is the opposite specialization and the best nuker in the game (w/ bad CC options), so they complement each other nicely.
Also, Xan can get to really high fire resists, eventually, if you give him the proper items. When over 100% resists in this game, the damage will instead heal that character.
Edit: As others have said, Neera is iffy. Wild Magic can be funny/interesting, but you need to be prepared to save scum a little for it (watch your gold after every fight). As a bonus, she will go back to the Inn, as she is one of the Beamdog companions.