Malgus, I think they co-opt them because they need tough guys, they need someone who is capable of doing tough stuff. For example they were something like army unit but pretty much free when it comes to terrorizing people, they were like protected from the ruling guys.
In exchange for that they do some hard fighting, and of course lot of pillaging. Yea of course there was also something like try to control them by giving them job and salary, but in most of the situations it did not work, they simply were too dangerous.
It was almost usual to have groups like that, they had weapon, skills, no morale, lot of cruelty… If someone had means to control them even for the short period, it meant power.