Huh? Why not just see a 2D movie instead?

Except it's not?
Interesting you should say that as I sometimes cite pool as being a game that I can play due to it being played on a 2d plane (compared to ball sports where the ball can be in the air, I think I'm underneath it but actually it will land 5ft behind me).I always lose at pool (again depth problem)
Yeah, that's why it's dimmer with the glasses on than off, but I don't see why it would be dimmer with just one eye with the glasses on than with two eyes with the glasses on.
No it wouldn't, the cinema 3d systems are passive and based on polarised light.because you're seeing 1 frame of image then 1 frame of black screen then one of image then one of black so on (the filter closed = black) so the effect will be darker over all.
I know how they work, you still aren't making it clear why it would be darker with only one eye. Each eye receives an image that is equally bright, as your eyes do watching a normal 2d film, close one of your eyes, does everything suddenly get darker? No, so why would it with only one eye and a 3d film?The glasses are polarising, and each lens is designed so that it doesn't let in all the light from the screen. That's how the 3D works - each eye only sees part of the image on the screen.