The interesting effect will be if people flock to the upcoming games that have trophy support, like the way that 360 owners literally jumped from 360 game to game in its first year that was a bit light on games to play.
If Microsoft was able to convince people to jump from game to game, even if the library, like all system launches, was filled with one or two good games and a lot of mediocre ones, then those people deserve the credit for giving the Xbox 360 such a high attach rate for games to console sales.
Anything that keeps a gamer coming back for more is a win-win situation, as long as it's not some super-expensive DLC. Too many games in my library go unplayed or unfinished, but I have to admit, I hang on to the Xbox 360 ones (and PS3 ones now) for the achievements and possible quality DLC. Anything that keeps me inspired to play again, or anything that can bring me back to playing with a fresh idea or set of levels, I'm all for!