Imagine my horror as I watched my student's mask spring a hairline crack and begin to fill with water. Kneeling in 35 feet of water, he looked rather confused as he attempted to clear his newly-purchased mask with no success. This was rapidly becoming a dangerous situation-- and it was his very first dive!
The crack in the mask grew bigger and it looked as if the mask lens was about to go. I reached into my pocket, whipped out my back-up mask, and signaled for him to switch it for his mask. He pulled on my back-up mask just in time, and the broken mask fell apart in my hand, not on his face. I am still frightened to think how the new diver could have reacted had the mask unexpectedly come apart on his face. Having the right emergency gear can mean the difference between a slight annoyance and an accident. For me, a back-up mask is required gear on every dive.
Back-up masks help to avert dangerous situations. We all practice swimming without a mask in the Open Water Certification Course, but how calm do you really think you would be if your mask suddenly came apart during a dive? Would you be calm enough to make a slow and controlled ascent to the surface? Would you be able to make the required safety stop from a deep dive? Realistically, I doubt it. Without a mask, reading your depth gauge or computer to determine your ascent rate and depth would be nearly impossible. Checking for boat traffic before surfacing would also be difficult. Perhaps you could find your buddy to help you. Still, do you want to go through a 3 to 5 minute ascent with no mask? Most open water instructors I know would not be very clear-headed in this situation. I imagine that most recreational divers would simply panic.
A diver carrying a back up mask can also help out a buddy with a mask failure. Dive guides and instructors, in particular, should carry a back up mask to be able to handle situations in which a clients mask breaks underwater. True, a single back up mask will not fit all divers, but a standard-sized mask will fit most people well enough to prevent them from inhaling water, and to keep them in control long enough to call the dive group or buddies together and signal an ascent if necessary.
Back-up masks are useful even in non-emergency situations. When properly prepared, a diver can trade out an annoyingly foggy or ill-fitting mask (learn how to test a mask for fit) for a back-up mask while underwater. On the dive boat, having a back up mask can save a dive if a diver's personal mask breaks or is forgotten on shore. I have used my back-up mask for each of these situations.
Purchasing and carrying a back-up mask may be a hassle, but consider the alternatives-- panic, quick ascents, annoyance, and missing a dive. Back-up masks do not need to be bulky or expensive; they just need to work. Most Buoyancy Compensators (BCs) have pockets that will easily hold a back-up mask without inconveniencing the diver.
In recreational diving, alternate air sources are necessity because we need to be able to breathe to safely reach the surface. I would argue that back-up masks are also a necessity because we need them to be able to read our gauges to ascend at a safe rate in a safe area. Get a back-up mask, put it in your BC pocket and forget about it. One day you or your buddy will be glad that you did.