*I* have never walked out of a movie early - because as others have said - dangit I spent the money and even if its worse than I expected I'll sit there and finish it! Thankfully, at least for me and my tastes, rarely has a movie been SO BAD I've even thought about leaving. Usually if its that bad I'll stick it out because then its just silly-bad.
Battlefield Earth was one of those. I went with a few others and we didnt walk out - but we still all agree its the worst movie any of us have ever seen. Ever. (Theater or otherwise). We were laughing and MST3King it by the end, but we didn't just leave.
My husband and a friend of his did walk out before the end of the first Fast and the Furious Movie. They made it to like the last 20 minutes and the theater lost power due to a storm - they decided to leave rather than wait the 10 minutes or so it would take to power the place back up and finish the movie. If they hadn't lost power they probably would have finished it, but once given the chance - they didn't want to and agreed it was horrible.
Watched Schindler's List in the theater when it released. When that movie was released a lot of local theaters got free tickets to give to local high school history class students and such (at least in my area) and so we all went by the dozens around age 15-17 to see it. I had a friend who had to leave during the "Ghetto Liquidation" scene - but not because it was a bad movie; she just couldn't handle the realness. She didn't return to the movie - saw it later at home. So again, not because it was bad, but because it was that good - and too much.
Same thing happened with a friend of mine watching the Rob Zombie Halloween I remake (which shows Myers as a pre-teen) - and he's still not sure why he had to leave for a few minutes. He grew up watching horror, same as me. We first bonded our friendship over the Halloween movies and our love for horror. He is a child psychologist - none of the movie was 'shocking' for him. But something about some of the more violent scenes with Myers as a kid, really hit him wierd and he had to take a 10 minute break in the middle - but he returned. And does like the movie. He just had to take a break that still confuses him.
So there can be other reasons to leave movies besides them being bad.