Incomplete doesn't mean invalid, though. I can say I didn't enjoy the first half so much that I didn't give the second half a chance. I will have enough information to explain why I didn't enjoy the first half and my opinion of the movie is completely valid.
I don't have to continue to subject myself to something I don't enjoy. I can explain what I don't enjoy. People don't need more than that to have a valid opinion of something. The only people I'd expect to have done more is somebody professionally critiquing movies.
Sure. But that's enough.
Demanding more than that isn't conducive to anytime but trying to make people feel inadequate for wanting to enjoy their time.
It's completely fine if you feel like you need to see the end of a story to determine if it was good, but not everybody is like that.