For me, the backup mask is a mandatory technical diver requirement and is a requirement for some agencies, like GUE, below the technical levels. I have lots of stuff in my drysuit pockets so carrying a backup mask doesn't bother me.
Below these levels, a backup mask is likely going to be used if you lose yours, or someone else does, when on the surface. It seems unlikely that anyone on a recreational dive will be calmly donating or receiving a mask unless they have been trained to near tech levels.
Something to consider in addition to, or instead of the backup mask, is replacing the silicone mask strap with a neoprene velcro "slap strap" to help reduce the chance that a failed strap will cause you problems above or below the surface. Many with longer hair find it much nicer than silicone straps, especially when not wearing a hood.
This is the kind whose velcro threads through the existing mask strap connections, and sticks to the surface of the pad. It's not the kind you can buy that puts a pad over the existing silicone strap, for obvious reasons.
My buddy has a slap strap that doesn't use velcro but plastic buckles, and the buckle closure got snagged and came off on a dive. I keep a few spare mask straps in my save-a-dive kit mostly for others.
Naturally, even a better strap isn't going to fix a mask that gets lost entirely...but the neoprene might help slow down the mask's descent so you can catch it in time.
