Switching to BUSS is irreversible
How so? Why couldn't you switch one (all) back on if the crew turned them off in error or when out of the condition which caused the UAS in the first place?
Why then is it required to keep one ADR On above F250
If you look at the QRH Unreliable Speed Indication tables for level flight in ABN - 34 Navigation, you will notice that above FL 250 the required pitch is 3 degs whilst below FL 250 it is 4 degs.
When clean, the middle of the green on the BUSS wants you to fly a pitch attitude equivalent to a speed slightly faster then GD. Above FL 250 the Mach No becomes more significant and I guess Airbus was uncomfortable with flying too slowly.