You guys are thinking in terms of PCs. In terms of consoles (PS3 especially) this is genius since you can get into the game, then grow generally weaker and then popup! "Do you want to buy the game through PSN?" It will also let you use the character data and saves from the demo, which is nice. It's genius for the PS3 because that console is notoriously hard to crack. Besides, if the consoles hardware DRM is cracked, they can download the game and play it anyway.
Any demo that lets me play the actual game, even if it's for a shortened time, is fine by me.