ginti
Contributor
I find a purge valve an interesting mask feature, but as a lifelong snorkeller and never a scuba diver, I don't regard it as a necessity. It depends how irritating it is for the wearer to have a smidgen of water underneath his nose. A downside of the valve is that it must be kept clear of sand or silt that might affect the valve's ability to exclude water from the outside and to expel water from the inside. Grains of sand may get in the way of the one-way diaphragm, leading to leakage, so the valve must be kept clean at all times for hygienic reasons too. Manufacturers often leave their purge valves sealed so that the mask wearer has the option to use it by unsealing it or to dispense with it, leaving the seal intact. It's really down to the mask user's personal preferences.
So I think it is not very common between divers because, when staying close to the bottom, the probability of getting some particles inside the valve is more or less high. This is true especially for well-trained divers, who may not lift sand from the bottom in the ocean, but are likely diving in caves/wrecks/other environments where a bit of silt/other suspended particles is always a hazard.
From what you say, I guess there are still some of these masks on the market...
Thanks again!