The fun in achevements is in getting them. 'Grinding' achevements can go screw a rock, but the good ones are those that encorage you to play in a way you wouldn't otherwise consider or a way that pushes your skills to the limit. I don't care about the boast potential, but thinking of them as extra objectives that increase the playability of a game is the right way to aproch these. A good exacmple is the 'pacifist' achevement in Geo Wars, which 'rewards' you by simply staying alive for a set amount of time, likewise the 'Dot Eater' from Ikaruga.
Conversly, CoD4 had pretty boring achevements, 70% of them were either 'complete this level' or 'complete this level on this diff', both of which I would do anyway in my standard playthroughs of a game.
Gamerscore/boasting is irrelivent, but if you don't like the extra objectives then nobody is forcing you to play them, dissing the system as a whole seems pointless.