What happens with that "absolute position relative to some arbitrary 0,0 picked by the mouse" when the user picks the mouse up off the table/pad/etc. and repositions it (i.e., they hit the edge of the pad and now "re-center" to continue moving left (or right) on screen). The mouse loses its 0,0 point reference as soon as it is picked up.
It could send a "reset 0,0" packet of some form in this case, but now reception of that packet becomes critical to continuing to properly communicate motion to the attached computer.