Here's a question I can't answer. The traditional Bayer sensor, as found in most DSLRs and compacts these days, has twice as many green photosites as red or blue.
My question: "Is it inevitably true that making each photosite as sensitive as possible produces the best image quality?"
What if each green photosite had half the sensitivity of the red and blue photosites? Well, some simple math (probably too simple) would suggest that the overall sensitivity of the sensor would be reduced by 25%. That's workable (c.f. Sony's translucent mirrors) but the benefit is that those green pixels would each take twice as long before they saturated as the full-well capacity wouldn't have changed. Some clever binning where adjacent green pixels could have their signal combined might help restore the green signal to noise ratio though the S/N ratio would still be worse than having green photosites with maximum sensitivity. I'm guessing that there would be a very slight loss in resolution in dimly lit areas of a photograph and a loss of colour information in the very bright areas although those bright areas would still have image detail that would have been lost had the green photosites been at full sensitivity and had thus saturated. Obviously some smart de-bayering would have to be employed to avoid colour tints in very bright or very dim areas of a scene but another guess on my part is that this is quite possible to do.
I generally find that if something as simple as my suggestion above hasn't been done there's a "gotcha" which means it can't be done. Maybe it has been done, or partially done, but nobody has admitted it? As to why one would want to do it, well, if the manufacturers are going to cram much more than, say, 20 MP into an APS-C sized sensor then dynamic range may suffer as there's less silicon available to store the electrons released during an exposure. This may be a way to retain more dynamic range as pixel densities continue to climb.
Kodak have gone the other way with some of their sensors and substituted clear lenses for half of the green ones in an effort to make their sensors more sensitive but has anyone tried the trick of making the green photosites less sensitive even if the reduction in sensitivity is less than the 50% I mentioned at the start?