Every edition's monster manual as far as I can tell has entries for dwarves, halflings, elves, etc., that have a similar listing as monsters. These usually include tactics for attacking the party, etc.
Does anyone ever use these? Do you ever use a warparty of neutral elves attacking the party as unprovoked as a clan of gnolls or a pack of wolves?
I think that my group would have major issues if I started using PC races as monsters. A specific named NPC as a villain, sure. Maybe even members of an evil cult. But just "these are generic dwarves attacking your group" - I don't see that flying.
If my view is typical, then why do game designers keep adding PC races as monsters? Beyond "this is what a typical warrior of this race looks like for comparison purposes" what good does it do to stat up a bunch of things that most players will never fight?
Ditto for good-aligned dragons, angels, etc.
Retreater
I wouldn't use randomly encountered demihumans in an unprovoked attack, but I would happily use them in a conflict of interests that might lead to combat. The stats are also useful for communities or patrols of demihumans who might fight along side rather than against the PCs.
The idea of evil/aggressive humans and demihumans is one I have used before, but not as random encounters. Humans can certainly be the villains and have lots of human henchmen and footsoldiers - why not demihumans? I think it was Keraptis of White Plume Mountain fame who had an army of evil gnomes. Guess what stats I would use for the rank and file of Keraptis' forces?
I think one problem is the presence of drow, duergar, derro and spriggans make some folks think that because those are the evil sub-races of demihumans, any normal-look demihumans are good or neutral. Big assumption that....