To each his own, I guess: the lower brightness is actually the main thing I like about 3D. It's occasionally done well enough to add something to the movie. Other than those cases, the only benefit is having not as bright a screen in a dark room.

I've read that some theatres reduce the brightness to extend the life of the projector light bulb.

I generally am not a fan of 3D, but my theatre seems to light 3D movies perfectly, while their 2D showings often seem dark.