I carry one in my dry suit, but most of my dry suit diving is not recreational.
When diving recreationally, I am usually in a nice, warm location with sunny skies and a wet suit with no pockets. If I were to somehow lose a mask permanently, which is highly unlikely (more on that later), I would follow Howard's lead and abort the dive.
When we prepare for emergencies, we need to prepare for them in a reasonable proportion to the likelihood that it will happen and the seriousness of the consequences if it does. I would be safer if I had a parka and snow shovel in my car while driving in the open plains in July, just in case my car broke down during a really freak snowstorm, but I take my chances and leave them home. I do carry them in December though, because both the likelihood and consequences increase.
Let's look at the likelihood that I will lose a mask. I have never lost a mask while diving, and I have never heard of anyone I know losing a mask while diving. I have had them bumped and dislodged, but never lost. If I were to lose it somehow, in most cases my buddy would be able to help me locate it. Thus, the likelihood of losing it is pretty remote.
How about the consequences? If I were in a cave or a wreck, or if I were doing a decompression dive, the consequences are serious enough to outweigh the remoteness of the possibility, so I do carry one. Even so, in my technical training I had to do decompression stops while both my buddy and I had lost both of our masks to our instructor's insatiable need for torture, and we still managed just fine.
A recreational dive is different. If my buddy is not able to help me retrieve the mask, then, oh well, might as well end the dive and get my spare ready for the next one. Even if my buddy is not able to assist me for some reason or other, I can handle a recreational ascent without a mask.
Thus, the remoteness of the likelihod of the problem, the lack of serious consequences, and the fact that I don't have a convenient pocket to stow it combine to allow me to dive without a spare and without a worry.
I am surprised by the poll results so far. When I entered mine, it said that 1/3 of all the responders were carrying spare masks for recreational dives. In the hundreds of recreational dives I have done over the years in wet suit conditions, I am not sure I ever saw anyone carrying a spare mask.