The reason that my lot (I believe and many others) use the MCP window for the first restiction is that if you use any other mode other than VNAV (and we accelerate the aircraft after take off in TOGA mode) then the aircraft will not level at the altitude selected in the FMC, it will climb to the MCP selected altitude. Hence it introduces a chance of an altitude bust... Also, not all climbs are made using VNAV, as when we are light we would see alot of TCAS RA's simply due to the climb rate, so vertical speed mode is often used.
Now, with the MCP altitude being downlinked to the controller, could we omit (that is if the aircraft is suitably equipped) the cleared alt bit in the initial call and simply pass the current alt and the SID?