As someone who runs a slower-than-average computer, I very much appreciate games that let me run at 10-15 fps, rather than slowing the game down to boring speeds or telling me I can't play. Telling me I can't at least TRY to play the game with some screen updates missing feels really snobby to me. Sure, I'd rather have it run at 30 fps, but at least I can for the most part enjoy the game at a lower fps. Games really don't become unplayable until they are under about 12 fps, so if you say that I have to run it at 30 fps or nothing, that's really cutting off a group of players who could enjoy the game (nearly) as it was intended.
Yes, games should give at least some cpu time back, and only hog as much cpu as they need :) Some games need to hog a lot of it though.
