First, check out http://en.wikipedia.org/wiki/Gamut for reference. The sample gamut picture in the top right shows a typical CRT--lets assume for the sake of argument that LCDs are similar.
If you add a yellow LED to that it just isn't going to add much. The yellow part of the spectrum is already fairly well represented.
*But* if they also change the hue of the green LED toward the blue spectrum then it has a good chance of really opening up the gamut.
The people saying RGB is enough don't understand chromaticity--go look for gamut plots of your favorite output devices and see how little of the full spectrum of colors they can actually reproduce. Printers are especially embarrassing. Your eyes can really see a whole lot of color detail.