Not actually true. RGB is modeled after the mechanism by which humans perceive colors (using red, blue, and green photoreceptors).
If you mix Red and Blue light, you still only have Red and Blue light, but the human perception system will perceive purple light, even though there is no EM radiation at the 'purple' frequency.
This is exactly the misconception I was talking about when I said that RGB is modeled after physical processes and not after human perception. There is no such thing as a red, green, or blue photoreceptor. There are three types of cones: L, M, and S; there is also scotopic vision with its own response curve. The L, M, and S cones respond to a gamut which cannot be reproduced with any RGB system that uses real primaries. Or, put another way, no RGB system with real primaries can specify all the colors we see.
Instead, RGB is a simulation of a physical system which uses three light sources: red, green, and blue. That's all it is. For example, imagine that you have three LEDs, or three phosphors, or three lasers. It doesn't matter. The point is that RGB simulates these kinds of physical systems, not the systems in the human eye.
On the other hand, Lab and XYZ systems are modeled after the perception of light. The experiments which lead to the creation of the XYZ color system had subject participants match the color output of an RGB system with that of a monochromatic light source. This experiment allowed us to use a well understood color space (RGB) based on a physical process to test how human vision worked.
You seem to mean: "a common engineering method of displaying colors to the human perceptual system"
while I thought you meant: "the physical properties of the visible band of the electromagnetic spectrum"
The same phenomenon happens in audio. The vast majority of musical synthesizers either model the perception of sound or the physical production of sound. For example, you can model a piano as a physical system with a vibrating string, or you can model it as a subjective phenomenon, constructing a frequency spectrum that sounds similar using FM synthesis, subtractive synthesis, additive synthesis, et cetera--none of which correspond in any meaningful way to the piano itself, we're really trying to trick the ear.
In the same way, I see RGB as a simplified physical model rather than a perceptual model, because it does correspond rather closely to physical reality, and it corresponds somewhat poorly to subjective reality. You can construct RGB as a simplification from a continuous spectrum model of radiation, all you have to pick the spectrum of your primaries. From there, you can use the RGB model in your physical simulations, such as ray tracing and photon mapping. Lab color does not work well for ray tracers because it does not correspond to physical reality: it is fairly nonlinear, and the coordinate system is awkward.
Likewise, RGB is a poor model for subjective perception (compared to Lab or XYZ) because its gamut is limited, and differences in RGB space do not correspond well to differences in subjective qualities. You can see how awkward RGB is for perceptual modeling whenever you use a color picker. It is frustrating to try and construct a pleasing palette of colors by dragging around RGB sliders, or even HSV/HSL sliders, because the model is so far from subjective perception that doing something conceptually straightforward, such as altering hue or matching luminosity, requires fiddling about.
In short, the description of RGB as a "physical model" is because we use it for physical simulations, as well as for working with hardware such as monitors and cameras. My description of Lab, CIECAM, XYZ, etc. as perceptial models are because we use those for modeling the subjective perception of color.
http://commons.wikimedia.org/wiki/File:1416_Color_Sensitivit... (note the sensitivities are normalized - in actuality the blue receptors are much less sensitive than the other two)
