I'm having a little difficulty with the distinctions being made here. Back in the day hirelings were fairly common, and were controlled, more or less, by the GM. NPCs are controlled by the GM, and if you have an NPC that sticks around a little more and a little longer than usual that shouldn't really be a problem. I think as long as they stay mostly in the background and don't ride rough shot over the PCs it should be fine.
I remember back in the early 80s no one in our play group ever wanted to play a cleric, so Cecil the cleric was a mainstay in most of our games, regardless of who was DMing at the time.
I think if you're careful, stay out of the decision making, and let the players feel like they have a valuable ally, there's no harm in essentially having a permanent NPC around. If done properly it can actually be a lot of fun, and GMs are supposed to be having fun too.
I'd definitely be careful and mindful of my players' feelings, but give it a shot and see how it goes. I'd love to hear if it worked out for you...