I've already said.
If you can't imagine how a self interested person wouldn't carry stuff round for someone he owes nothing to then you'll just have to accept it without understanding the rationale.
If he's so self-interested... then he should be interested in getting a pay-check at the end of the day... And if that means carrying some stuff I'm pretty sure no matter how self-interested you are you'll carry some shit.
Even Tony Montana did a couple shit jobs before he became a drug lord.

For game play reasons, it is to differentiate the mercs from the other two groups even more. If you want people to mule for you, get slaves, if you want people to fight get mercs.
I really don't know how to reply to this. except, "Why do you want to differentiate them more"? Mercs are trained killers (well... not all of them exactly) who now await their next pay-checks, walk to hell and back to fight for their leader if the money's good, and yet they're apparently too good to carry some stuff?
Slaves are jet-addicted brain-washed junkies, who you can either use for general menial labor, sell-off to the highest bidder, or sic them on some ass holes giving them the hope for another dose of they succeed.
There was already a huge difference between mercs and slaves. Do you ever see gangs using slave mobs to attack their enemies? No. you see some loner cheap-o's using it. and it rarely turns out good unless it's just grenade spam.
And zubrowka. Stfu and gtfo hm? Quit trying to suck Developer Balls will ye? Opportunist.