In contracting the company or person hiring you generally knows very little about programming and even less about yourself. You have the information advantage,

In contracting the company or person hiring you generally knows very little about programming and even less about yourself. You have the information advantage, you usually price the job, and you know how hard you're going to work and how long these sorts of jobs usually take you. It makes sense in that context for you to carry the risk if you choose to be paid by deliverable. Whereas, if you're a trucker, you don't know any more than the company does about whether the gods of traffic are going to take a dump on you that day - and there's not a whole bunch you can do about it. In that context it doesn't seem to me to make much sense to pay on that sort of basis. The point of paying with a target is - at least when you're not trying to screw people out of money - to tie reward to the risks that they can control. If they can't control the risk, it becomes little more than a way of concealing from people what the take-home is going to be.