Tom Goldman said:
Certain gamers [http://www.escapistmagazine.com/news/view/104447-Worlds-Biggest-Gamerscore-Breaks-500-000] have been known to seek out, borrow, and rent titles they wouldn't otherwise play just to get a quick boost to their Gamerscore.
I've skimmed the thread so far and no-one seems to have bothered to point this out. He did not state it specifically, but Trinen's problem may be with the gamerscore being artificially inflatable by playing useless shovelware with easy achievements.
This is a very Nintendo way of thinking. They don't, and in my opinion really shouldn't, trust every single third-party developer to make a worthwhile achievement system that doesn't just reward players for completing the first level. Nintendo, being the control freaks they are, would not want a point system they create to be so open to manipulation and generic mucking-up by lazy third-party developers. That's assuming they don't have problems with the idea of a point-based system in general, which they might.
That would also be why Nintendo has no problem with per-game achievements (and, in fact, seems to enjoy implementing them). If you want to brag about your achievement in SSBB, you can do that, and your friends will see you got it by hard work (or not) in that particular game. Compare that to an abstracted gamerscore, which you could have gotten by playing Barbie games. Yeah, people can look through your achievement list, but it's still obfuscated and, thus, easily manipulated.
Finally, making a system-wide 'achievement' point system puts pressure on developers to implement achievements lest they look like they forgot to do so. Not every game NEEDS achievements, and that pressure sometimes results in the lazy "completed level 1!" achievements in many XBLA games. So, I can see where Nintendo is coming from. Perhaps the compromise I would like to see is per-publisher achievement scores. So a Nintendo score and an EA score and the like... but that would be confusing to new players, and Nintendo is very into attracting new players, so it's unlikely.