It's a good idea, but it's also prohibtively expensive for me when I can pay, in some cases, the same amount or less for a complete game.
I mean I could buy new DLC for Mass Effect or GTA 4... But given the cost, I could pay less money and get both Braid and Castle Crashers. It's just a hard to sell me a few new missions in a game I've had my fill off when there are so many games I've not even played yet.