I think this could turn out to be a really cool idea. Of course the npcs would have different styles of combat, depending on what you want. Or there could be an option for you to set the follower/leader to be more stealthy, rather than running into enemies hacking and slashing, and you can change it whenever u want.
There could also be a "mercenary for hire/follower for hire" profession, where npcs ask you to accompany them on their own quests, they lead, you follow/assist, and you get paid for your service. It would be more fun for you to RP a mercenary character.
I think this would be an awesome idea. Anyone agree?