i'd be really interested in bruce block's opinion on this!

I'd love to give my opinion Mike... thanks for asking. So here's the major issue:

RGB seems like all you need right? Well... remember that in order to separate out the light so that each pixel only registers the proper spectrum you need to use filters in front of the ccd sensor. When you filter out certain parts of the spectrum you will inevitably filter out some of the light that you actually want to pass through which leads to problems. Some real world examples of how this works are the differences between Fuji and Kodak color film. Chemical emulsion film stocks have to filter light the same way essentially, 3 layers and filters, so compromises have to be made. The conventional wisdom is that Fuji is good for high color saturation and reproduction of blues and greens, whereas Kodak is good for dynamic range and reproduction of reds and yellows. The easy way to remember it is to look at the colors of the boxes the film is packaged in.

So the fourth color filter may allow for the sensor to make up for the weakness in the blues and greens... key word is "may." In your RGB CCDs, there are 50% red pixels, 25% green and 25% blue as I understand it. The "emerald" just gives more weight to that neglected segment of the spectrum. It doesn't matter whether the color "emerald" is "base" or not, all that matters is that you know what the effects of color mixing are. Painters don't just use 3 colors to create the entire spectrum, theoretically possible perhaps, but not very practical at all. Really, in the end, it's the image that matters. Forget all the theory and look at the print... and on that note:

http://www.dpreview.com/news/0402/04021306dimashootout.asp

I mistakenly wrote the pixel ratios for the Sony RGB CCDs above... they are actually 50% G, 25% R, 25% B.

Actually, this makes a lot of sense. Our eyes are much more sensitive to green. Here's a chart on eye color sensitivity: http://www.amastro.org/at/ot/othcs.html

My favorite book on the psychology of vision is Eye and Brain, byt Richard L. Gregory.