Even Shawn Wildermuth admits to being a gamerscore *. The whole point of gaming is the satisfaction of winning. And what a great way to enjoy winning is to get a reward that lasts forever. I can go back in my gaming history and almost re-live the moments be looking at my cheevos. I have proof and I can compare against other gamers. What if I'm talking to a group of people and I say, "I got 5/5 on all the dance central 2 songs on hard" Then some other dude say, "Yeah me too". I can then say, "Oh yeah, what's your gamertag. I'll look it up on my phone". Gamerscore is very important to me and to many other people.
As for Xeets, I could be wrong about how great they will be, but we will find out soon.