Sulam is basically correct. I was born with strabismus (my eyes don't focus on the same point). To avoid perpetual double vision, my brain compensates by basically only using one eye at a time. It switches automatically when I'm looking to the side, and I can also switch consciously. (My color perception is very slightly different in each eye, it's weird.)
It turns out that stereovision is the least important of several depth cues humans use, and is only really effective out maybe 10 feet or so--beyond that the parallax is too small to give much useful info, and we rely on apparent size, relative motion, surrounding context, etc. I can catch a ball just fine, as an adult.
It's really only an issue within arm's reach, and then only when I'm distracted. Occasionally I'll reach for something without paying attention and miss by, like, a foot. Also, I can't use 3D glasses. (With the new polarized kind I can at least wear them and see a normal 2D movie. With the old red/blue kind, everything would be either red or blue, depending on which eye I was using.)
