I don't understand why people have such a problem with gamerscore. Take me for instance, I love gamerscore. It gives me something to compete against others with. The OcUK GT database has a leaderboard and trying to climb up it gives me enjoyment. When im playing games I earn gamerscore that helps me go up he leaderboard. I can then get further enjoyment when I see my place in the leaderboard increase.
If it wasnt for achievement points, I wouldn't have done some of the tricks in Crackdown, played through Condemned without using guns or tried outlandish 40yd efforts in Pro Evo 2008. Also im currently playing Bully, and without achievements I wouldnt see the point in collecting rubber bands/giving students wedgies but because I get something (20 points) from it, I might as well play the game to the fullest.
I dont think that because I have a fairly large gamerscore it means Im a better gamer, but it does mean that I've gained more enjoyment from my games in most cases.