I don't really see the point when I can just look at the histogram. Of course, my D50 can't display an RGB histogram. It only displays a "green" histogram and passes that off as an overall luminosity histogram.
The only situation I can think of where this could be useful (off the top of my head) is if you're shooting something....say, a tree or flower or something, and there isn't a lot of green in the photo, then your camera is likely to underexpose since 50% of the pixels only pick up green colour, so you're better off overexposing on purpose to make up for the lack of luminosity that you'll get from just the red and blue channels. Or maybe I misread and misinterpreted the information. I read that post while working at uni.