I agree that achievements can be disruptive in single player games that are trying to build atmosphere. I can just picture the message "Crowbar Frenzy!" popping up in Silent Hill 5...

But many genres aren't trying to be that engrossing, so achievements work fine for them. Perfect examples are sports and racing games. Beating some track record or scoring enough points in a certain amount of time and getting achievements for it would just add to the sense of accomplishment. It's the same thing with multiplayer games. Beating your opponents feels good, but beating them and getting a message about your impressive kill streak is even better.

I know what you're saying though. Having just killed a big monster in GoW, I got a message saying I just killed the big monster... Seems redundant and takes you out of the moment.