Both Newgrounds, and Kongregate have them. Who else thinks it is time for Armor Games to step up to the plate? It would help with point progression also.
While Armor Games would be more like Kongregate and Newgrounds with achievements, a lot of games on AG already have their own built-in achievement systems. Having a site-wide achievement system would not hurt the site in any way, and can only improve it. Just because there are achievements doesn't mean that people will only play a game to get the achievement points. Look at the Call of Duty games on Xbox 360 and Playstation 3. Most of the achievements/trophies for those games are based on the story, but most people will keep playing the game once they get the story achievements, or not even get the story achievements at all.