So an ATC cleared level may not be the level you are intitially cleared to....at least not until you reach a point on the SID.
This will cause mayhem with certain (non-'native english' speaking) operators. Why change what already works? Why jam up the (already overloaded) frequencies with ATC having to issue longer clearances when wanting you to ignore SID restrictions? How much time/distraction will ATC have to give when someone mis-understands their clearance and climbs into the stack?!!?