I was preparing to revisit the history of the martini, particularly the part
about when dry vermouth had to be specified in place of sweet vermouth, but
you've answered things beautifully above.
I'll add that I think this No Vermouth business is attributable to two other
things, at least the recent trend:
1) Trendiness - people feel that much more cool when they can ask for a drink
such as a martini and then even specify that they like it a certain way.
Nowadays people seem to think that asking for no vermouth makes them appear as
martini aficianados, as though they know the REAL way to enjoy this drink
2) Vodka martinis - I find that most of the people giving me this "Extra Dry"
line are ordering vodka martinis. In that case it actually does make a bit of
sense as vermouth was originally added to gin and is far more suitable for
complementing and mellowing out the complex flavors of this spirit. Since
good vodkas are virtually flavorless there really isn't much for the vermouth
to do in the drink, other than add a slight vermouth flavor. In a gin martini
it's a much more carefully designed balancing act of flavors.