Achievements add to a lot of replay value, which I approve of. I'll beat a game, then play it again to grab those achievement points. I can't tell you how many times I've played Dead or Alive 4 online just to try to get that A rank.
However, multiplayer achievements suck for the most part. What really sucks is if there's an online multiplayer achievement that you're trying to get, yet you happen to be the only person on the planet playing that game online.
I'll look forward to Sony's trophies when they implement them for games I'll actually play. I'm really surprised they didn't do it for MGS4.
Jam it back in, in the dark.