It's not so much that most of our field of vision should be light or dark as that we have a bias for light=detail/important, dark=background/unimportant.

This has a slightly perverse effect when a computer monitor is placed in a real environment; looking around my environment (sitting in my cubicle at work) my laptop draws my attention like little else because it's so damn bright.

Maybe websites bias towards light backgrounds because it discourages users from getting up and doing something other than browsing.