On Friday, 15 December 2017 00:00:23 UTC+2, John Foster wrote:
Why do so many gliders seem to have multiple varios? Is it that critical to have a back-up for this instrument? Do they often fail and leave you in "
the lurch"?

I removed my mechanical backup vario when I upgraded my electronic vario.
My backup vario is XCSoar on an Android phone in my pocket. Sure, it's uncompensated but I've flown with uncompensated varios in the past and it's not that hard if you maintain a constant speed while thermalling.
If both fail and I can't stay aloft then I'll park my glider in a farmers field.
The same argument applies to relying solely on GPS vs using a compass and map as a backup. Where I fly if I get lost I'll just land out. No sweat.

I can understand people having backup instruments if they fly over hostile terrain with few out-landing opportunities.