SimReality: They can impose the same limit on the number of achiements as they do for arcade games. Plus, who cares if somebody gets 10,000 achievements if they are all 0 points?
For instance:
What if I make a game that just immediately gives 10,000 achievements to the person playing it? What if I also put obscenities in the description of these achievements? I distribute this game, and everyone who is fooled into running it gets their gamertag spammed with all this junk. Now not only can they no longer find the achievements from the real games that they want to play (they'd be buried down the bottom of a ridiculously long list) but whenever they go to play some other game, people will be sending negative feedback about them because of the obscenities that appear in their gamercard. Not exactly a great experience, and not at all what Live is supposed to be about!
That's just one example: there are many more such.
This is not to say that zero point achievements aren't interesting, but it is far from obvious how these could be made to work in a safe and secure way.
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