> because we were sure we needed drop shadows and geometry transforms for windows

As screens get larger, the amount of pixels you need to push to composite windows gets larger-squared. It makes sense to move the pixel pushing away from the CPU and more importantly away from CPU-RAM and on to a separate RAM bus.

The "single buffer with invalidation" model of Win16 (I cannot remember how it works in X) saves memory at the cost of more redraws. The composition model allows you to do things like drag window A over window B without forcing a repaint of window B every frame.

It also allows for better process isolation. I think in both Win16 and X11 you could just get a handle to the "root window" and draw wherever you wanted?