I think you've hit on the main limitation of computers playing "dumbed down". They've gotten better (HIARCs and I've read good things about Stockfish), but generally a computer doesn't know how to play mediocre chess like a human. The way it dumbs itself down is by making periodic blunders and then returns to master level play. That's why playing against a computer is so different, and ultimately less rewarding, than playing a human. Of course if you have something like HIARCS where you can choose from about 100 common openings, it can certainly be a timesaver for opening prep. More of my $.02 from the man with many pennies. 
EDIT: Oops, I didn't really answer your question. I think it depends on the programming. Sometimes they blunder early and you get such a good position that you can win against master level defense. Othertimes it makes a minor blunder and then just digs in and you can't find a way to beat it. The old Chessmaster used to be fun to play against because your computer opponents would have different styles and frequency of blundering.
Hey, I'm curious to know whether people generally perform better against computers. For example, I would usually lose to a 1,500 player yet I've found the 'hard' computer setting, which has been roughly that rating, to be quite a bit easier. Is it normal to find them easier to play against at this level? They do make some strange sacrifices that don't pan out sometimes.