for not wearing helmets on commanders, most have a shield of some kind or zealots and believe the emperor protects. yes it reveals them, but you can expect with that technology of the future, hiding wont be much use, as most races have means to wipe out entire planets.
if you wish to know more on 40k lore, i now a lot as i loved the universe and games for a greater portion of my life.
and looking forward to watching this lets play.
