Seems bizarre - I could see maybe if they're used to shooting with live view on a tripod? That's the only time I really use live view... not that I shoot much in the way of portraits. It's definitely not as stable, and if you have a fast lens on a bigger body, with a flash on too, you're really going to want that weight up close rather than at arms length...
The LV accuracy in focus may be an argument when using DSLRs, but shouldn't apply with a mirrorless camera - as that's using LV all the time.
Or maybe they're just hip video photography tutorial recorders who don't know much about the principles of photography