True, but I wouldn't do that. If I start to watch a movie, I will finish watching it, no matter how bored I am, because for me the ending and the final impressions left by movie can change it into something that was definitely worth watching, even if it was two hours of boredom at the time.
So what does this mean? It probably just means that in a perfect world, there would be different ratings for "hated it, so I turned it off" and "finished watching it, hated it". Imagine an extreme example where a movie was rated like this:
95% of the viewers: Hated it, turned it off
5% of the viewers: Hated it, but finished watching it, eventually loved it
Assuming I also fall into one of those two categories, it obviously wouldn't be the first one, as I would always finish watching even a terrible movie. That means that I would end up in the group of people who eventually love the movie and would be really glad I sat through it.
However, these are probably the only ratings I can see:
95% of the viewers: Hated it
5% of the viewers: Loved it
So I probably wouldn't bother with that movie, because I would think I would be unlikely to love it when 95% hate it.
Alright, this is an extreme and contrived example, but my point is that you should be able to decide whether you want to see only reviews from people who finish watching the movie or everyone who's sat down to see it, even if they only watched the first 15 minutes. The first option would be for me, the last option would be for someone who still thinks an hour and a half is wasted if you're bored to death, even if it's the kind of ending you can't stop thinking about for the next two weeks.