Whenever I game without the ability to earn gamer score.. it doesn't feel the same. I feel like there's no point to playing the game, but only when it's a game that is on the Xbox. This is the reason why I hope Windows 8 let's you PC game and earn gamer score.
I don't have an Xbox, but I can still gain points. Certain games developed/publish from certain companies will be using the GFWL and/or Steam program. In turn you still earn Gamer Points.
While Steam only records achievements based on the individual games, and can compare achievements to the global community without a point system in general. MS pools the achievement points based on the number of achievements made based on the game(s) played. Now, I have not been on the GFWL/Xbox community to compare achievements and points.
EA, also decided to join into the gaming community bandwagon with their EA Origin program, time only tell if they decide to start a achievement/point system.
Most of Gaming user play games for gaining the more score with respect to their competitor. I also think that game score make gaming more exiting and without the games score the enjoyment of game would be decrease.