Bookworm, you will indeed find that an increasing number of airports the requirement to switch it on when pushing back an leaving it on until after parked.
I'm sure someone from LHR ATC will confirm that the Surface Movement Radar uses the txpdr code to track the aircraft on the ground.
Futhermore at LHR, we change the code after parking to 1000. I believe that with Mode S this will allow ATC to determine the registration when under tow etc, even when the txpdr is off.