I expect it's just a function of electronics in general, not anything specific to a GPS.

Most of the time my GPS is just a nice addition to my landnav loadout. But there are times when the GPS is more critical, like when I'm exploring an area completely new to me and I have very important waypoints programmed in that I want to find. Examples would be water sources, unique campsites, critical trail junctions, etc. In those cases I take a second GPS, which isn't too hard since it's a Garmin 401.

This is why I keep my old GPS units after upgrading.