Its pretty much covered.. Their intent was to give new/different goals to the existing games without having to add content.. Win for the players as it gives you more "goals" to accomplish, IE why we play games in the first place. Win for developers as it gives the end user more "game" without significantly more development time.
It sounds like your core argument is against the quality of achievements, well that's up to the developer, not the platform. You could say Microsoft is partially at fault because they mandated a certain amount of points implemented per game. The mandate itself is probibly ok, but the amount may be too high.. I think that, coupled with some not-so-creative developers are the reasons for the call-it-in lamer achievements that are all too common.
Still, I think the system overall is a good thing. They don't hurt anybody. *obligatory* If you dont like them, then you always have the option to ignore them.. ( I think you can even turn the notification off ?)
Personally, I enjoy working on individual achievements, and comparing with friends who happen to be playing, or have played the same game. On games I really enjoy, they give me an excuse to re-play to achieve new goals where I would have otherwise had to put them down. On games that suck, they get ignored just like the intended content itself.