I agree. A demo disk for a game you're buying is not a bonus. A demo disk might get me to try/buy the game, but if I'm already buying it, I hardly need a demo, esp. a short one.
One of the best things Sony did, imho, was the Jampacks and demo disks. More than once I bought a game I played on a 5 buck Jampack, that I would not have bought had I not tried the demo. Of course, that works the other way too.