Well one of the benefits is that the affiliates who does push the program has a much more exclusive program to push and therefor have a chance to make more money as the program and sites doesnt get oversaturated..

Also an advantage is that you get to weed out the riff raff affiliates that sign up to every program in the world just to get access to the free affiliate promotion porn...

The small squeaky affiliates that always complain and require help you can reject at the door so you dont have to waste time trying to help them get a business going ...

A disadvantage is that if you dont land big affiliates its hard to make sure that you get enough traffic for the program as it might get drowned out by the tons of webmasters promoting something..

Should also be easier to controle chargebacks and fraud as its easier to check the sites that referre traffic and catch people using shady tactics to drive surfers to the program..

just what i can think of on the top of my head..