Half-breed races aren't really necessary or important to the game, but they should be available as an option in the standard settings, given that half-elves, half-orcs, and whatnot have been in fantasy novels since Tolkien at least.
My Rhunaria homebrew has no halfbreeds except for the rare instances where a dragon or outsider mates with a mortal (dragons are immortal in Rhunaria, BTW). Most such half-immortals (mortal but with longer lifespan and limited traits of their immortal parent race) are shunned or outright killed on sight by normal folk, before they even have a chance to grow up and become dangerous. In the campaign, the only instances of halfbreeds appearing were two half-dragons, one half-gold and the other half-red, at different points in the story; both were PCs, but the half-red dragon half-human PC didn't stick around very long. The lack of half-breed races in Rhunaria hasn't had any negative impact. Though, orcs are a playable race in Rhunaria, mind, so half-orcs aren't needed to fill the 'strong but dim-witted' archetype. Rhunarian mortal races are simply incompatible in genetics and magical essence, when it comes to mating.
Quick Edit: Oh, and yes, Rhunaria is a world where most intelligent life evolved rather than being created. Only dragons and a few other highly-magical creatures may have been directly created by divinity to some extent. The world's dense magic accelerated evolution. Cats evolved into the Nari catfolk, who then later evolved into the other humanoid races over time. Much later, a few humanoid races were created through magical transmogrification of existing races, and such is how yuan-ti, dark elves, sea elves, kobolds, lizardfolk, minotaurs, and the like were created.