I have only lost a mask once, and it was when my buddy and I were rolled by a 20 foot wave on an exit (long story--happy ending). We were on the surface, and it did not influence our prospects for survival.
When I was making parascuba jumps for the USAF, we wore our masks backwards, with the strap in front (sometimes in our teeth) to ensure we did not loose it during the jump.
I have now been solo diving for about 55 years, and had only that one incident. I have on occasion worn a spare mask, either in my BC pocket or on my left arm (that actually is a pretty good place for them). But mostly I don't have a spare mask. I dive rivers, and any drag in current is not good. Sometimes the current is fast enough to flood the mask; this has happened several times, and a close-fitting mask is helpful. But if the mask comes off and is lost, it's no big deal in my diving. I went through the U.S. Navy School for Underwater Swimmers, and during the pool harassment portion, my buddy and I simply gave the instructors our masks and did the entire session without the masks. Cupping the eyes to get a bubble is something I learned in the YMCA lifeguard class in the 1950s. It's one of those skills for a bored lifeguard trainee who is floating for a half hour at a time. I see no reason to use a redundant mask for most of my solo diving, which is river diving in less than 30 feet of water.
SeaRat