a) All the information on the PrimaryFlightDisplay, and others, is important at any time of flight - especially during the approach to land after ~12 hours ! Trust me, manufacturers and airlines do not provide instrumentation that is not required !
A comparison might be a car suppressing some of its instrumentation, for example its anti skid or brake warning lights after it had been driven for more than say 4 hours?
b) It would be condoning fatigued pilots in charge of airliners, which is a definite no-no.
There is a very simple way to make sure that pilots do not get confused by PFD's: Proper selection, Proper training, and no fatigue.