I don't think they ever will. I think they see it as taking away from a simple, "It just works" user experience. The reasons that I see it taking away from the experience they want are:
1. It would require users to think about how much room they have on the device. Yes, you theoretically have to do this anyway, but with approx. half of users never, ever connecting to iTunes, that is something they would have to begin worrying about. Apple doesn't want people to have to think.
2. It wouldn't be fast, especially for big files. Soon, people would be complaining that their multi gigabyte movie rip is taking too long over the air. I've done this with a jailbroken phone over wifi-it's a pain. People would try it over 3G, or worse, EDGE.
And two non user experience reasons:
3. It's scope creep. A phone or tablet isn't meant to replace a computer, despite what everyone says. There isn't a solib business case for them to spend the time to develop this or add in additional storage for all the garbage people would acquire.
4. It's another possible attack vector.