I would say there could be two things.
Current flat panels don't handle color levels finely enough, and since the eye is very sensitive to yellow (it's flat in the middle of the rainbow, meaning the center of the spectrum our eye can discern), this lack of fine setting can be seen - adding a 'yellow' channel may just allow the screen to output more precise yellow levels, making the image look more contrasted. That wasn't the case with CRTs, due to the much more precise levels an electron beam could be set at.
Actually, right now the information carried by high definition streams allows the reproduction of colors that the eye can't really see well - so, with proper filters, it should be possible to have images that actually benefit from this process.
The second thing would be (it can actually combine with the first point) that it would allow a better interpolation on the subpixel level: while square pixels require three side-by-side very rectangular subchannels, adding a fourth one may actually allow for a square subpixel grid. Now, any of the three primary complimentary colors could have fit the bill: magenta or cyan. However, magenta is at the lower end of the preceptible color spectrum, and cyan at the higher end. Yellow is flush in the middle. So, back to point one.
It is a good idea. However, the reason why I'd never get one of those screens is because, well, subpixel font rendering would be screwed up by a non-RGB grid...
Don't let that fool you though, one reason for the gig is that, seeing the screen from the side allows us to see how thin the screen actually is...