RGB is confined color space, and adding yellow would greatly improve the accuracy of the display. It would allow the monitor to display CIE L*a*b color. The "L" channel stands for luminance; the "a" channel is green and magenta; and the "b" channel is blue and yellow.
Theoretically it covers the entire visual gamut - that is, all that our eyes can see. While it may not be entirely noticeable in a medium like TV, you can bet that photographers will happily buy the monitors.