The vast majority of my dives are cave dives....so I ALWAYS carry a spare mask. Scootering a kilometer or more to get out of a cave without a mask would not be fun...
I've only needed a spare mask once...and that was on a 90m wreck dive. My mask broke(when my dumb ass ran into something face first inside the wreck) at 25mins bottom time. The rubber skirt tore and it flooded. I switched to my backup, exited the wreck, and did my deco with no further incident.
Mike