I have not used Cable TV or over the air TV for years. Everything I watch is either purchased on iTunes, or streamed from the plethora of networks offering shows to watch online.
I would welcome this feature, as many of the iTunes purchase I have made are TV shows that I have only watched a few times (unless the show is amazing).
I'm in the same boat (Satellite-free since October 2008). I'm a bit worried about Hulu going to a pay-per-view or pay-per-month model sometime next year ... I watch a good portion of my content on non-Hulu sites, and if I needed to pay a dozen different sites a subscription fee I'd rather just not watch anything. Go back to buying DVD box sets.
In any case, a good subscription service from Apple, assuming they are able to bring most of the majors on board, would be a no-brainer for me. As much as I like what Hulu is doing, it's interface sucks (unless you look at it relative to the other Flash-based sites out there, whose interface sucks-squared). Having a subscription delivered directly to my AppleTV for smooth viewing (or even just into iTunes, although that's a 50% solution compared to the ATV) would get my money every time.
IMHO, Apple could go after several different markets here (not necessarily all of them at once, though):
1. The "catch-up". The main problem with episodic TV shows is that you can't just "jump in" and watch the latest episode. Hulu is a complete fail here after late-October: the "last five episodes" of whatever series extending back to the third episode after the pilot is not a good way to "get into" a series. Apple could allow viewing of episodes back to the start of the season.
2. The "browse". You don't know if a TV show is worth spending $2-3 on until you've watched an episode or two of it. I've made a few "conversions" from watching Hulu of a show to buying it off the ATV because I want better quality. However, I'd never have just "bought" that TV show were Hulu not there to give me a "preview" of what the show is like and where it is going. One possibility here is that Apple allows no-fee downloads of the first, say, 5 episodes of any given series (perhaps within a given season), then if you want to continue the series you pay a subscription fee just for that series.
3. The "mainliner". The watcher who wants to completely envelop themselves in a show. Provide links to online content directly from iTunes, etc. I don't know. I generally don't geek out on TV shows like this any more so I'm not the best one to ask about this business model.
4. Largest market: the casual viewer. This is the guy that will turn his TV on at the same time every week (or pre-program his DVR to tune to a channel at the same time every week) to watch the TV shows he wants to watch. Although this is the largest market, I think it is well-served by $85/month cable and satellite subscriptions. Apple will have a hard time cutting into this market without first making a "name" for itself on one of the above markets, I believe. But, it's another market for them to be thinking about.