Why can't you just watch it on TV? You are already paying for it by TV license, subscriptions and/or adverts.
I wouldn't (in a million years) pay for a TV program when I already pay for it through my cable provider.
There are many reasons why renting TV shows beats the heck out of trying to find what you want on cable/satellite. My wife and I starting renting TV shows on DVD from NetFlix years ago and it's a great way to watch TV shows. Here are a few of the reasons:
1. No commercials. You can watch three "half-hour" episodes in just over an hour (they're about 24 minutes each with credits, so maybe 22 minutes if you skip commercials). (Obviously this is not such a big deal if you have a DVR, but it's still a deal; see next point.)
2. They're uncut. When TV shows go into syndication, they always cut a couple of minutes out so they can put in more commercials. And did I mention there are no commercials?
3. You can watch them in order from the beginning. My wife and I have gone through several TV series all the way through. Some shows make more sense when you watch them in the order they were intended to be aired (instead of haphazardly catching them on TV).
4. Not every TV show worth watching is on TV. For instance, there's a lot of British stuff that's really well done, but not routinely shown.
5. It's hard to beat the value. One NetFlix rental can give you up to eight episodes/disc. Is it better than free (or already paid for)? In my opinion, yes (for the various reasons mentioned and some I'm probably forgetting).
In my mind, iTunes rentals has no chance of beating out NetFlix until they offer TV rentals at a reasonable price. Buy them for $1.99/episode? That may be reasonable if you want to own them, but it’s outrageous if you just want to watch them once. With NetFlix, we average over 8 DVDs/month for about $18. That would only buy 9 TV episodes from iTunes, but for renting it can be 20, 30, 40 or more episodes (depending upon the show and how many episodes are available per disk).