Been a while since I flew the Airbus, but:
- It's probably policy on any modern aircraft that you don't rely on what MCP/FCU is showing (button lights, values), but rather on what your PFD/FMA/ND are showing. If there was a wiring or some sort of connection problem between MCP/FCU and other parts of avionics, the selected value/mode could potentially not be transmitted to other computers. Never seen it for real, but better safe than sorry.
- I believe 1-2-3 is so you remember their positions, should you ever need to switch them off in flight, as 3 is in between 1 and 2 as I remember.