let's put it like this: if I were only blurring pseudo-hires pixels it would be a very bad hack (and I would have rejected it).
by equally blurring all 512 pixels modes, it is still an hack (and as such is turned off by default), but not so bad anymore imho, because it still describes the way TV would output the image

now, concerning general filter support in the core, it's not like there are thousands of devs which could add the necessary support into the core. I believe there are maybe 2 or 3 which could, and probably they won't care about incorrect and driver-specific workarounds, if they ever come to implement the support...

in the meanwhile, it's like with other screen artifacts: driver-specific workaround are probably better than nothing.