I'm interested the uses of 3-pin bayonet cap or BC-3 lamps in other countries. Where I am in the UK they're most commonly used with the 80W and 125W mercury vapour lamps, this was because the first mercury vapour lamps could easily be mistaken for incandescent lamps and the BC-3 cap was used to prevent them being installed in a standard bayonet socket.

I've also seen them used in electric fires for fireglow bulbs and more recently on CFLs used with special pendants to prevent the use of an incandescent bulbs.

I'm wondering which areas of the world also use this socket? I'm guessing it's almost unheard of in the US?

If i remember well, the use of bayonet socket in UK is the relic of the incandescent lamp patent dispute between Edison and Swan (Swan won in the UK) one and half century ago...
As Edison had no such "issues" in other countries, his screw-type socket spread everywhere, only except UK...

I think lamps for public areas and/or military uses were for some time equipped on purpose by similar, but incompatible sockets (e.g. E28 instead of E27 base), as in both cases thefts were quite common and cost a lot to fix it and/or cause hazardous situations before such fix.
Using incompatible sockets make these bulbs not usable at homes, so it was expected, then lamp thefts would cease. However i do not know, how successful this "tactic" was in reality...