Excellent read. I enjoy how you manage to bring psychology and cultural bias into perspective as regards how we the viewer view these things. From a literary standpoint, those things are often forgotten or overlooked in the creative process and can mean the difference between a believable character and a not so believable one.

For myself, I dislike the helmet-less little guys because I despise painting faces. I'm decent enough at it, but would rather see the faceless death machine instead. Also, I (like the outside viewer that I am) wonder why one wouldn't wear a perfectly good helmet if it's available. I used to just assume that the Sergeant/Officer, being a higher priority target, might already have taken a round or two to the face/head and ditched the now useless chunk of metal in favor of actually being able to see. From practical experience, a helmet (non-far future powered one of course) can restrict vision and hearing. On a busy battlefield that can spell disaster. a Sergeant needs to be able to direct his soldiers with as much info as possible.
However, having said that, I agree completely about the psychology behind your rationale. It makes absolute sense to the point of "duh, why didn't I think of that?" I suppose its because I am not an Astartes.