Apple would have complete control over the software they wanted to offer on a theoretical Mac App Store. Not so different from the retail boxed software they sell at Apple Stores. I don't think it's on the top of their priority list but I bet it happens eventually. If you look at Steam on Windows you see there is a proven model for success with desktop app stores.
But Steam's DRM system actually provides some value to the customer: it makes for a very effective anti-cheating system. If you are detected cheating, you get banned at a random point within a few days (to kill your testing cycle), and you lose your purchase. The only time you ever see cheating is during free-pass weekends. I normally pirate everything, but this is worth paying for.
To most users, a general purpose computer is a power tool that they'll never make full use of. What most people really want in a personal computer is a backplane for various appliances. They just want to buy those things, install them, and have them just work. If an App Store ecosystem could mean Apple could completely shepherd users and take care of their every need, then this would add value in much the same way. And Apple would be able to figure out how to market this and get paid handsomely. This is the exactly the direction that Apple is going with their retail stores. (Especially with the revamp to increase the social interactivity.)