I keep a spare mask clipped to my backplate. If my mask gets knocked off, I can just reach behind myself, unclip the spare, and keep going.
As an exercise, while I was diving a spring, I took off my mask and tossed it away. Then I started looking around to find it. I searched and searched and didn't have much luck. Finally, I signaled my buddy that I was ending the exercise. They retrieved my mask from the bottom, handed it back, and we continued with other skills.
After the dive, we discussed the lost mask drill. I noted that I had no problem (other than being a bit less comfortable than usual) being underwater without a mask. I noted that I couldn't see well enough to reliably locate and retrieve my mask, and I certainly could not see well enough to read my instruments. I *was* able to catch a glimpse of my SPG and read my computer thanks to the "bubble mask trick", but it was too difficult to be practical (at least without much more practice).
Then my buddy told me something that was rather eye-opening (hehe, sorry). Apparently, when I took my mask out and tossed it away, a little boy who was out there snorkeling saw it, swam down, and picked it up. My buddy had to go over and retrieve it from him, at which point the mask was tossed back down as I continued to search. What did I take from that rather unexpected complication? Simply this: Things do not always go according to plan.
What are the odds that a passing barracuda will see your falling mask and swim off with it? Probably not much, but what if someone turns a corner, finds your mask, and swims off without seeing you or even glancing around? I decided that having a spare mask was a reasonable concept.
Of course, I'm a DM and also a solo diver. As a DM, if I'm mauled by an inexperienced diver, I don't want to be impaired and searching for my mask. With my spare, I can pop it on and stay on top of things (and hopefully be able to see and retrieve my original mask). As a solo diver, while I'm fully capable of ending a dive with no mask, it seems much preferred to just slap on a backup and find my original mask while I'm still there.
I've been kicked, kneed, elbowed, slapped, clawed, landed on, launched through, rammed, shoved, run into walls, and even quite literally *marched* over by an entire OW class as they proceeded into a *cave* (*sigh*). Never once have I lost a mask, although I've had mine knocked askew numerous times. Carrying a spare is so utterly trivial, however, that there's little point to even questioning whether it's a reasonable idea.