A spare mask is more of a consideration when you're 3 miles from daylight in a cave, than it is 60 feet on a reef. A spare mask strap in the save-a-dive kit is plenty enough redundancy for most people's needs. Spare masks should be carried when losing a mask could be life threatening, not an inconvenience. Losing a mask in a cave or wreck penetration could be life threatening - losing a mask or breaking a strap during a shore entry, that's just inconvenient.
I actually have a spare mask, but would never carry it - it's just my old one I leave in my divebag because I recently bought a Cressi Big Eyes mask, and I figured that keeping the old mask as a spare would be more convenient than having to buy a new spare strap for the new mask. If you only have the one mask already, and you're not doing penetration dives, a spare strap should be enough for your needs.