Let's face it, those things are addictive. I can't help but smile when I see the little 'Achievement Unlocked' icon, and can't help but check my gamerscore after it appears. Achievements are just fun, and they are the main difference between games released for the 360, and games on the PS3 and Wii (aside from stellar online play). So why isn't Microsoft pushing that fact?
The way I see it, the achievements are Microsoft's chance to push even more games and systems. Think about it, how cool was it to play Halo 3 and get a new Helmet or Armor just from an achievement? How fun is it to discover you have a new GamerPic just by unlocking an achievement in Hexic? Microsoft could use this 'secret weapon' to push even more games.
Right now, I have a Wii and a 360, and will eventually get a PS3 (MGS4), but if I have a choice between a game that is available for all three systems, I immediately go for the 360 version, just because of the achievements! The GamerScore also makes me play games I wouldn't have normally even tried out back in the day (Maddens, King Kong, etc.), but I'm discovering how much I was missing by playing these games and love some of them (Viva Pinata!!!).
Now you say "Well, that's great Hektic, but I don't care about my gamerscore, so I want to see the uber-graphics of the PS3 or use the Wii-Mote to rock out." This is the base that Microsoft can reach out to! Alright, say you wanted Guitar Hero III for the Wii to check out Wii's online play and you liked the thought of not having to use batteries (The guitar is powered by the Wii-Mote), what if it had been announced that when you completed the Whammy-Mania achievement on the 360, you unlocked a hidden Iron Maiden song not available on the other versions? I can see something as simple as that would make gamers want the 360 version.
All of their big games have done it to some degree; Halo and Gears stand out in my mind. How cool was it when you set the picture you got for earning "Seriously..." as your gamerpic and everyone was like "OMFG You got Seriously?!" Gamers work hard for those achievements, and while they are rewarding just to get them (COD4 anyone?), I see them as an untapped opportunity for Microsoft to reach out to an even larger gamer base.