I was watching some U-tubes about Cody Lundin. In one of them I noticed that he went into the desert with some trainees and carried what looks like a meassenger-style bag. I wondered : Isn't a backpack more comfortable ?
I can hazard a guess: A messenger bag will make you much less sweaty than a regular backpack. Which probably is a huge plus in a desert environment. (Although I've never been to a desert I do know the sticky feeling of carrying a backpack in hot weather).
Personally, I wouldn't carry anything heavy and bulky in a messenger bag. YMMW. But I can understand why anyone would want to avoid carrying backpacks (unless the load/bulk makes it unavoidable).