"Gated communities"......

[davem]

Tolkien's original intent was not to 'invent' a mythology for England, it was to 're-discover' England's lost mythology. Shippey goes into this in great detail in Author of the Century. Tolkien was taking the last remnants of the myths & legends of England & attempting to reconstruct the original, rather like reconstructing a lost language from a few scattered inscriptions.
He took what was there. Elves in Middle Earth live in underground realms, because Elves & Faeries in northern European myth live in underground realms, like the Hollow Hills of Scots & Irish mythology, or the live in the Wild Wood, a la Robin Hood, which grew out of the stories of Puck & Robin Goodfellow. Dwarves live underground in the Mines they dig their ore from.
Tolkien is starting from this point, not from the point of social commentary. The Legendarium develops & Tolkien explores the motivations of each race, trying to give an explanation of why they live the way they do, & comments on those motivations, but his starting point is the original myths. The comments are secondary, & ultimately do not provide an explanation of the individual races way of life.
If the races live in separated communities its ultimately down to two things, room & psychology. There's room in Middle Earth for each race to live as they do. Psychologically, the differences between races are immense. They couldn't live together in true harmony. Elves are, for Tolkien, 'embalmers'. They want to turn the earth into a work of art, put a frame around & 'freeze' that perfection. The Dwarves see the earth as a source of resources for their craftsmanship. Men want to dominate the earth. These races could not live in harmony, & luckily, there's enough room in M.E. for them not to have to.
Each race's realm reflects its essential nature, taken to an extreme. I can't see them as 'gated communities', at least not in the sense that Tolkien was 'commenting' on such things. Maybe 'applicability' comes in here, or seeing in the Legendarium what you want to see, but I think its dangerous ground, because I can't see that it had any part in Tolkien's thought, & therefore any 'comments' anyone sees Tolkien making on the subject will be sheer guesswork.
The subject of 'gated communities' may have a great deal of relevance to twentieth century humans, but its one area where I don't think Tolkien has anything to say.