Besides, if you haven't finished a movie, you can't claim to have a valid opinion of it.
I'm sure you didn't mean it to sound this way, but this kind of gate keeping sounds very elitist. I can have a valid opinion of a movie I didn't finish.
Usually, me not finishing it means that through the portion I watched I was disliking it so much I decided finishing it was a waste of my time. It doesn't happen often, but it does happen. I don't need to see the ending to know i didn't enjoy the journey. The onus is on the creators of the movie to ensure I enjoy or am at least engaged throughout and I don't, nor does anybody else, owe it to them to see though the time I've set aside to watch it. Especially if they could be bothered to hold up their side of the bargain.
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.