For any backup to be effective, it must be in the scan at all times.
Don't think of it as backup, think of it as redundancy. The GPS is on,
the pilot is monitoring it. It is in the scan at all times. If there is
a disagreement in instruments, the pilot must decide which ones are
correct and which ones are incorrect and take appropriate action. All
done smoothly and within the flow. Any one system can fail, and the
pilot can maintain control of the plane. The likelyhood of two systems
failing at the same time is small, MUCH smaller than 1/2 the likelyhood
of one system failing if the systems are designed to be indpendent.