Lets say you agree to mow the grass. Should he pay me upfront? No. When the job is done. AND you've done a good job, he's not going to pay me for mowing a maze on his yard...no matter how graphically impressive it would be simply retarded. And a waste of time. He'd toss you out. Its the same logic we are doing with companies. Sure we can get it for free. But are you going to appeal to us so that we feel that you've done a good job to be worth being paid? Or should we toss you out on your butt for a half-done job.
Unless we're talking about MMOs, then the grass mowing analogy is incorrect. Games are products, mowing is a service. I could certainly see refusing to pay someone if they did a bad or incomplete job mowing a lawn, but I don't see how the same recourse system could be applied to games or any other product while maintaining a viable business model. If all I have to do is say I don't like a product and that frees me from having to pay for it, what reason would any company have to make products at all?
Now, if a game does not actually work, or it causes harm to my computer, I believe that is justifiable grounds for me to box it up, go to the store, and get a full refund. If this is the kind of scenario to which you referred as a half-done job, then obviously I agree with you.