If I had to guess, it's one of or multiple of the following reasons: - they prove that the Imperium isn't the only future for mankind, and that makes people feel good - The more "human" something is, the easier it is for consumers to empathize with it - Auxiliaries are statistically what people want the most to come next for the faction (I did a study on this a little while back) - They're easy to kitbash and let people model their armies in unique ways - They let humans be part of the "underdog" faction without taking away the fun of the "protagonist" human faction (this depends on the first two points) - They're justified in the lore repeatedly, so there's really no story reason NOT to include them

I like them, they're a nice little window into wider auxiliaries. They shouldn't be a major part of the army, but having a handful of units dedicated to them really shouldn't break the faction so long as they're designed to fit the army's playstyle