How many critical components do we rely on?
1. The engine - now if you have only got one, its pretty critical,
2. The pilot - ah yes, pretty critical and there is only one of those as well,
3. The prop - not a lot of good when part of a bade detaches,
4. Even the throttle cable - it concentrates the mind when it snaps and the engine stays where you last put it,
5. Etc
We love system duality. That is why we usually have two fuel pumps, and two mags. There is a chance of the GPS failing, or the MDF going blank, but there is not much chance of that happening at the same time as the backup GPS in your bag failing.
Simple, by all means rely on GPS for navigation, but if you do, go to the small expense of taking a spare with you.
I bet if you do the engine or the pilot or the prop will quit long before both GPS units quit at the same time.
Now how much redundancy do you want?
As for big boys flying instruments with GA glass cockpits these days about the only nav instruments you are going to be left with when the sh** hits the fan is the magnetic compass so best you check the backup GPS in your flight bag works.
