I can only imagine this is the actual preference of two types of people:
1) Those who've never had a good martini made with high-quality, fresh vermouth, carefully chosen to complement the gin (or vodka, for that variant) being used.
2) Folks who've adopted the preference not due to genuinely preferring straight gin to an actually good martini, but because they've decided it's somehow more hard-core, classy, or old-school, due to Churchill et al. (I wouldn't say it's any of those, but to each their own—if anything, the Churchill thing reads to me more as an alcoholism joke than serious advice)
I would agree, however, that given a decent-or-better gin and cheap vermouth that was opened six months ago and has been sitting in the back of a cabinet since, I'd rather have the gin neat or over ice. Both primary ingredients need to be good to make a tasty martini—otherwise, sure, why bother, and yeah, the vast majority of bars probably don't go through enough good vermouth to justify stocking it at all (let alone multiple kinds to pair thoughtfully with various gins) so many places just can't make a good martini. I certainly wouldn't judge a bartender's skill at anything but a very-fancy bar based on how good a martini they make. It's not a drink with an acceptable cheap version, IMO—there's nothing for the cheapness to hide behind.
I'm in category 1 personally. I'll drink straight vermouth (if it's good) as an aperitif. But so many people seem to agree with Churchill I find it funny.
