You know, this gnawed on the back of my head for a while and I've finally realized what's wrong with that.
A color monitor is of course designed to work with the cone cells in your eyes, with the three color guns corresponding to the three basic colors of light that said cones perceive. (And on the output end on the monitor the three colors are presented as a tiny little triad of subpixels.) If you just sum all three together and stuff them out as additive luminance on a monochrome monitor then you're also going to make for a pretty bizarre display. For example bright white will be three times brighter than, say, bright green, which is not how our eyes perceive a normal color monitor. Black-and-White displays are designed solely to understand luminance, and what you'll be feeding it is a three-way compound of luminance, chroma, and hue. So, yeah... it will look really weird.
I'd say just wire it up as it is, with the luminance line on blue, and force OS X to monochrome. (You can do that via the "Universal Access" settings control panel.)