Chiming in to agree with the "smurf the Fade" sentiment. God damn was that crap ever annoying.

Also, I play on whatever difficulty is fun to me. Sometimes I like a challenge, sometimes I don't. If a game increases the difficulty by making AI better or whatever, then I'm more likely to want to bump up the difficulty, as opposed to "congratulations, it now takes 50 shots to the head to kill someone!"