Yeah, but it's not like you're every going to play it again after you finish it. It's sort of like a rental service for DD games since there is no netflix for non-disc games.
But yeah, the price is only worth the amount of money saved compared to buying games you would have actually paid for. In this case I would have paid $15 to play Stacking for a few days and never touch it again. So $3 more for a dozen other "free" games for 3 months doesn't seem like a bad deal.