I have always thought hiring NPCs would be cool. Especially since there are not enough players to fill that role. I am not a big part of the Forced interaction with other players all the time anyways. Indeed, let me list the reasons why I would rather have hired NPCs to guard me than players some of the time:
1. Not always on at the same time. You would have to waste time every time you went on trying to hire new mercs.
2. Unreliable. "Sorry. Gotta run. Have to take out the garbage/go to work/sleep/eat/getting bored/want to finish that quest."
3. Sometimes you just want to log in and have some quick fun without having to deal with player drama. I don't care if my merc's parents were eaten by red-eyed possessed Kran and that is why they don't trust them.
4. Playing as a group of you and a few NPCs is STILL roleplaying. roleplaying is not always player to player interaction.
5. It is not robbing from the game. Maybe from YOUR way of playing, but it would be adding to others' way of playing.
6. Snorks.
7. Good minions are hard to come by sometimes. (hey, hired guys don't always have to be good.)
8. More NPCs to kill! Yay!!!!
9. Less bothersome to try to figure out how to pay.
10. They would actually DO what you told them to (well, more often that players, anyways)
11. More realistic caravans. (the NPCs could be hired to hold/carry your garbage to the next town to sell.)
12. The list could go on with much more advanced options, such as having scribes, apprentices, or even being able to switch between computer controlled characters in your group, but that is another thread for another game.
13. It would be fun. 'nough said.