Yeah, this is one problem I think could be addressed with iTMS. I know I would be much more likely to buy new artist singles when they are released--therefore creating even more buzz for that artist's upcoming album release-- if I knew I could get a credit toward the purchase of the full album when it comes out (if I decide to).
In the same way, if the TV downloads are to be really successful, there should be a similar system. Obviously this is just in its infancy right now, so there isn't really any precedent, but I would like to be able to pre-order the whole season for the discounted price and then download each episode as it comes out. Or, once I have paid for enough individual episodes to equal the price it would be to buy the whole season, then the additional episodes should be free. Come to think of it, that should apply to music as well.