I had the same question during rescue class. The only time that I have see a pocket mask, not to be confused with a spare mask in a pocket, was during rescue training. It makes you wonder why they teach it. While it would probably be a good idea to carry one, theres only so much room in my pocket. I think the surface marker and reel are more important so they are in my pocket. Based on my informal survey, very few people own a pocket mask and I have yet to find anyone that carries one on a dive.
Mike
P.S.
For anyone that doesnt know what a pocket mask; from the DAN website, a pocket mask is Used in conjunction with rescue breathing and constant-flow oxygen, the Pocket Mask helps you provide additional oxygen to the non-breathing injured diver.