First, iTunes reviews of both music content and TV/Movie content are incredibly fruitless for the most part. For one, as you pointed out, much of the ratings have a great deal to do with the overal usablity of the store, ie: rent vs. buy. Often great songs are given low ratings if they're simply not great enough to be free in someone's opinion. Or you see poor ratings because the discovery download's genre is country and the weekly freebie is also country (this week). If you can somehow sort through all of that you get one, maybe two great ratings. A thoughtful rating that weighs in various aspects of the song/tv show/movie.
Unfortunately iTunes and their rating system will be the sounding board for all things wrong. If the iTunes free show/song isn't quite free by a certain time on Tuesday the users let Apple know. Apple most likely doesn't bother checking the ratings and why should they? Likewise, if an artist is relying on Apple's ratings to really showcase true untapped talent then I say MySpace would rule over this; and that is saying quite a bit! Unfortunately I believe iTunes should have rating systems and ways to comment, but that also means you have to take the good with the bad.
Now you posed a question based on an observation of the clearly failed rating system or the poor use of such system and that is whether or not studios should open their catalogs up to digital download. If I were a studio I would not make this decision based on the fact that some user posted in the rating of that movie their discontent with the lack of availably to buy. Likewise, I believe studios still believe people like to hold things in their hands. Like me, it is possible that they're not yet convinced that digital downloads are worth it. They could also believe that it could be a gamble. Provide the movie to buy in stores, produce the same number you would normally produce (however they decide that) and open up the DVD for digital download. What if they made 10,000 copies of a movie and historically speaking they sell all 10,000 copies. Now they've offered up digital downloading and they only sell 5,000 copies of the physical DVD. What do they do with the other 5,000? Obviously it is not an exact science, but it is something to consider. Right now it could be a studio is not sure what magic number is for DVD production if they open up digital downloads, they only know they better provide at least the physical copy because not everyone is on board with digital dvd download. I know I am getting there, but I am definitely not there.
So does the constant bitching about the lack of ability to buy a DVD and only rent it on iTunes mean the studios will reconsider? Possibly, but I think the whining and crying about the lack of a purchase option is worth not losing your silk boxers over cost of over-producing a new release.