Another advantage of maximising the light collection surface is making the AA filter less necessary to avoid aliasing (moiré), because the sensor itself averages the light gathered in a wider area. As a side effect this averaging acts as a LPF.
Regards.
I believe that similar filtering (~100% fillrate) can be had by using microlenses, with the side-effect that your f/1.2 lenses will be more like f/1.4 lenses?
Anyways, this kind of boxcar filtering is a crude prefilter before sampling. You need to have additional filtering (lens diffraction, scene/camera movement) in order to suppress aliasing by a lot. Fortunately, increasing pixel counts means that diffraction/movement is filtering more on a pixel level.
-h