When all three ADRs are switched off you go into direct law. Ever tried high altitude flight in direct law? This is non reversible. Leaving at least one ADR on allows ALTN law, and recovery of systems if/once Icing blockage clears.
Below 250 the thicker air make direct law flying much easier. It also allows time to get the aircraft sorted for the landing.
This is only for the unreliable airspeed case.