RCP,
In general 2 servos in 2 channels is preferable because of the adjustability -- adjusting the end points, etc.

Never having used the miracle Y i can't say how it performs or if it picks up the difference.

The lag between channels with standard servos is almost unnoticable, but it does exist. With digitals, the 9Z WC2 has an accelerated frame rate which resolves that to the point of being unperceptible, but nothing else i know of on the planet does.

So the answer is, standard Y harnesses are far from perfect, and so the norm has become two channels. Also a y-harness is one more piece of equipment to put drag on the system and to have potential to disconnect or fail.