I can see how that could come about (lumping window manager concerns all into the one bucket), but to me it's security implications make a good case for re-architecting this part of the system. I.e. having a separate part of the OS which marshals access to the visible desktop when transitioning between ACPI states.

For hardware-accelerated compositing situations, it might have something to do with unnecessarily long / out-of order retention of the GPU frame buffer, which I could conceive of as being dismissed as inconsequential in most circumstances.