I may be in the minority here, but I think the above analogy is flawed and this might actually be a decent idea.
Try: You go to a Ford dealer and test a car. It goes like it should. You say "Eh, let me sleep on it" and come back the next day. Then you test drive it again, but this time the salesman thinks you're just trying to drive the car around town, so he gives you a shorter test drive, perhaps not on the interstate (thus limiting you to 35-45mph). You say "eh, I'll sleep on it" and come back 2 days later. This time he limits you to going around the block.
I don't know about you, but I play demos once, maybe twice. This is to prevent people playing demos repeatedly and not buying the game. See Zero Punctuation and mirror's edge (?). He says something to the effect of "Just play the demo repeatedly and you'll have the game." It's to stop stuff like that.
Besides, couldn't you just reinstall the demo? Delete the demo and delete your game save and redownload it (sorry for people with sucky internet connections). Thus your demo will be renewed. If you're willing to go through that hassle, fine keep playing the demo without buying the full game. I think a lot of consumers will be buying the game.
For Gamestop and other kiosks I suspect Sony will give them special demos that don't degrade if Sony ends up implementing this (regardless of whether or not they get the patent).
