They just work better during assembly and even more so on disassembly. The screwdriver needs very little forward pressure to stay fully engaged and for the most part just doesn't slip.
They work well in all sizes, and it's fairly easy to insert a Robertson screw with one hand when used vertically and even to a certain extent when used horizontally (as the screw balances readily on the screwdriver).
With a small screw and brand new bit, sure. Bulk box 3 1/2" construction screws and a bit that's seen 100+ screws? Not without a magnetizer or magnetic bit holder.
As I understand it they are tapered and this is supposed to hold the screw on a bit, but a little bit of wear fixes that right up. I think they do hold a bit better than a Phillips even when worn but it's not night and day.
No one makes a decent Robertson screwdriver outside of Canada other than Klein. I think electricians in the US use Robertson, too.