Apple isn't going to add any kind of subscription or rental (yes, I get the difference) for iTunes music until there's a good business reason to do so. Right now, the Zune is eating up sales of every other MP3 player, but isn't touching iPod marketshare. It's doubtful that the Zune ever will.
MSoft's Zune music store offers buying/rental options, plus a ton of other features that iPods don't have. But nobody seems to want Zunes, anyway. Because all of those options lead to confusion. Some tracks can be burned to CD, some can't. Some will "expire" sometime in the future, others won't. Who wants to keep track of all that? What could be worse than firing up the MP3 player, only to find that the song you have a jones for suddenly blew up?
The iTMS is successful for one reason only. It just plain works. It offers a better buying experience, period. Adding rentals, or even subscriptions to the mix would have to be done very carefully to avoid ruining this experience.
I had a subscription to Audible.com for a while. And I liked it. But I dropped it eventually because there would inevitably be those few months where I didn't find any books I wanted, and then I felt like I was paying for nothing (even though I had gotten books cheaper other months). The same problem exists with Netflix. Once in a while I get too busy to watch a movie that I get for three weeks or so. And then I realize that I just paid $15 to watch that movie once, when I could have bought it at Target for $12.
I don't agree that there's no market for buying movies. The DVD sales business has been doing fine for years now, despite rental being available. Not everyone buys movies, but enough people do for Apple and the studios to make a lot of money. I agree that the quality must improve, and adding the special features would be a good way to increase sales. But I don't think that adding a rental, or even a subscription, option is by any means a "do or die" proposition for Apple. I think they're just fine for now with purchase only.