I can tell you from my own experience since MP is so heavily promoted by LL it has a lot more exposure than an avarage in world shop unless you spend time, money, thoughts and resources in letting people know where you are.
Clearly KC succeeded in this and as such it pays off.
If I was to run KC I'd rather "die" than maintaining a sim AND give the labs a percentage of the food etc. sales via MP.
Unless I was to calculate that percentage and added that as extra fees to the price of the products for the customer so they had to pay extra for that convience.
The fact they sell starters there makes perfect sense every exposure of their product even on MP makes that more people know about it I see that as an advertisement.
It just makes no business sense to pay fees for a private region and pay fees for MP.
Linden Labs can't have their cake and eat it too.
I know they would love to though
EDIT: I think MP sucks, is laggy, unreliable and anti-social.
And many merchants I know which had little exposure in world indeed gave up their sim and went to Marketplace only.
