There's an intangible draw in them that other genres experience. AMXs vs. Javalins; Yenkos vs. Camaros; etc. As said, they represent to top of the food chain of a model promoted as an individual's statement, a performance high in a metal shell. Their distinctiveness goes deeper than appearance to a pedigree rooted in competition, the automotive equivalent of hand to hand combat. Almost universally, they were originally bought by an enthusiast, and used in the arena (at least the earlier models). It's uncommon that there's not a story or more each one carries with it. Isn't that all a legend is, a story of triumph, victory and just plain making you feel good about yourself. In one, you become part of that legend, a legend that's still being written.
P.S. After writting the afore paragraph, and reading the other posts, I have to say the little weiner syndrome statement is short sighted. There's plenty of folks own own one that's not anatomically challenged. As people are, the ones most likely to throw stones are the ones must quilty of the crime. The most prevalent theme, or touch, used in restomod Mustangs are pieces taken from the Shelbys-- stripes, scoops, gauge pods and aprons. Copying / mimicry is the sincerest form of flattery. They're just frickking cars. In any forum, there's going be be some that blindly follow the flock, be it for investment or prestige. It's as ugly and narrow minded to slap a universal characteristic to ownership as it is to race, color, nationality, etc. There's going be some who blindly cast stones.
I took it not a question of value, but of emotion. If emotions were easily defined, why are we continually coming up with more ways to express it in word, song or visual art? I can no more explain the emotion a Shelby evokes versus another Mustang, than explain emotions in general.