Dark text on a white background is a good rule for print-based design where you're dealing with light reflecting off a surface.
But when designing for media that use light emitting from the source, the rules are different, and a lot of people don't recognize this. How many TV shows or films do you see that have their credits rolling up the screen in black text on a white background? Web design is no different, really. A bright white background tends to "glow" which can infringe on the letterforms, making legibility and readability suffer. The contrast is too great.
I would suggest that people who think black-on-white is more readable in on-screen are just psychologically more comfortable with it because they are more comfortable reading ink on paper. But from an optical standpoint, it isn't always the best choice for a transmitted-light application.