The problem is it's ok for a experienced pilot that can reach across and set a squawk, but for a student that may write the squawk down, then reach across to set, then fiddle with the altimeter, all the time there's radio silence where everyone is waiting to hear a reply.
Or read back the squawk and pressure, then standard reply, then set squawk...what time has been saved, especially when given a shared squawk.
If anyone remembers the Upper Heyford method, seemed to work well.
Call, given squawk.....pause, then just asked destination, everything else was seen on the screen.
I think they could tell we weren't a F111.