For some applications you want "every frame is recent" and can tolerate some glitches as trade off for not having perfect frames. Also displays have different pitch values, subpixel arrangements, color spaces, etc. A line from A to B might look completely different on different monitors. By offering only RGB arrays as abstraction Wayland pushes the the need for care of those differences down to clients which is a mistake.

Additionally a Desktop system in general is much more than a bunch of bitmaps blittet together. Applications need to interact and such functionality has to be tightly integrated into the compositor with standardized protocols otherwise it will be impossible to have such functionality. Taking screenshots, drawing to the root window or knowing about coordinates of windows from other programs are some examples.