I'm not sure as to the underlying calculations, but I certainly use my prisoner haul as a recruitment reserve when the conditions are right. If there are no immediate threats, either locally to my own force or say to one of my settlements requiring speed to get to, then I haul them around with me until I get what I want out of them. If I need speed then I either sell them at the nearest town if reasonably close and quick to get to, or just dismiss them if I already have plenty of cash anyway. It really depends on the situation but more often than not I'll drag prisoners around with me for a while until I've topped up or get sick of the drag.