I can't say I've ever needed the spare once a dive started. However, it's been handy as a loaner on boats more than once. I try to keep a reasonable Save-a-dive kit with me, but on some boats it's just not feasible. Having it inside a pouch that I'm bringing anyway doesn't really cause any extra work, so I figured I would just leave it there.Man I am seeing a lot of gear here... A spare mask? The only time I ever lost a mask in the water was when I was rescuing some people in a flipped canoe.