It would work great for consumers-- if you play more, you get more games to play.

But it would be a terrible idea for the business. "Spent" achievement points don't show up on a company's profit reports.

Right now, Microsoft is winning on both sides of the achievements equation-- it costs very little for them to keep the Achievements system up and running, since they've already built in a tracking system, and it obviously (as they say in the article) drives both sales and rentals.

Turning Achievement points into some kind of currency wouldn't do anything but cost them money. Sure, it would give you free games, but none of the major videogame companies are in the business of giving away free games.