Sorry to change the 'direction' of this thread, but I have my own opinion and a suggestion.
Achievements [to me] are more about my personal achievements than something to use as bragging rights. To be honest, though, I dont own an XBox [yet] and I'm not 100% sure how achievements affect anything else. Still, all the PC games I've played with achievements (Portal, Team Fortress 2, etc) have posed new challenges to me within the existing realm of the game, and therefore add a certain amount of 'replayability' - even if its just for my own satisfaction.
So, what is stopping you from adding achievements to your game? Sure, you cant add 'official' achievements to your game, but you can definately put your own system in place, storing each user's personal achievements on disk between plays. Even if you [read: your players] dont get anything else by obtaining these achievements - it still adds value to your game. Doesnt it?