Well, I'll share my point of view.
First off, you need to remember that this $0.99 rental price includes HD, where actually buying HD shows costs $2.99. Bigger difference, especially for someone who only wants HD content.
Secondly, for me, I only buy shows that I know are great, shows I want to keep in my library. I do not want to buy a show I have previously not watched, with there being a chance it might not be good. I just don't want crap taking up precious storage.
This works well for me, because there are many shows I'd like to just watch instead of own, and since I don't have television, and even if I did, I wouldn't pay the $15/$20 for HBO or Showtime, where several of my favorite shows reside. So yes, I'd happily pay $0.99 for an HD pilot of a new show, to see what all the "rave" is about. Not only that, it's instant and it eliminates any need for physically leaving my house and renting a show. It's nice.
The last point; I watch shows on my Macbook, since I travel consistently. So iTunes rentals very much apply to my lifestyle.
Of course, this is just me. Many of you pay monthly for cable, therefore you have absolutely no need for this service.