Using independent channels for servos (on a good computer radio) allows you to independently adjust the center, end points, direction, differential, and even speed of each individual servo, so that all surfaces work in unison.

Using Y-jack does not give you this kind of control.


Another cool thing you can do with a high end radio is adjust the 'curve' of the servo/control surface. The servo arm moves in a circle and thus the speed that the control surface travels is not linear. With a fancy radio you can change that - make it linear or any shape you want (smooth from low to high, more sensitive at mid, sensitive at low and mid but not high, etc.)