well, as byuu explained once, it's part of the way a real tv would diplay snes output: a tv would not be able to separately show the 512 pixels present in hires or pseudo hires mode (i.e. no alternate columns as in the first screen, but only a blended mix of the two as in the second one).
iirc, it's something related to phosphors in a crt tv, but I guess he can better argument the need for this (completely unrelated to ntsc filter, though)
of course, this will be an option in the Driver Configurations and won't be present for NSS or bootlegs in MAME... and it will be marked as an hack in the code (it is also pretty much isolated in the last step of the drawing, so it does not obfuscate the way we document SNES drawing behaviour)
EDIT: also, let me stress that this "averaging" code is not a filter to improve the appearance and make the games look better, but it is the best way to make the LCD output similar to the expected tv output when the SNES outputs 512 half-pixels in place of the usual 256 pixels... still an optional hack, and marked as such, but without altering the way things should appear.