It's the pet NPC thing all over again. Which never ends well for anyone.
I dunno, I've done it a couple of times, using them to pad the party, actually, I had a couple NPC's that I created and the players want to keep them around. Then again, I do give the PC's full info on their stats, and let THEM roll for the characters, maybe that's what's different? I dunno, it's best done in short bursts, I think, and they should not overshadow the PC's.