Upcoming update: Inquisitor Crusader acid spit... that is why!

But seriously, usually sergeants or above simply don't use their helmets as it's an identifier for them while the troops should remain uniformed. Though I will agree - something is missing.

Instead I propose implementing a new slot: Iron Halos for Crusaders and Psykers - Helmets (or cowls?) for Assassins

Iron Halo: http://wh40k.lexicanum.com/wiki/Iron_Halo#.UmiZgXBworU

It is an age old question, and few can seem to find a definite answer.  High rank Space Marines (as well as most Imperium leadership) are not seen using helmets.  It does inspire the troops.  Other types of protection exist, and many of the advantages can be gained from the cybernetics implanted in later ranks.  Some even claim that Space Marines are so enhanced that the skull is already hardened like a helmet.

So it is a cosmetic option.  The Devs may offer it as an option.  Or someone might make a modded file that allows it.  Time will tell.

So +1

I have noticed when equipping T2 armor the headgear does change so perhaps headgear/helmets will be included but at higher tier levels?

At T2 all variants that I have used appear to have a thin skull cap type with some eye wear aswell (to imitate targeting augmentations perhaps?)

I would like to see armor and helms separate though, not only would it enable more varied visuals but could also be another way to give varying stats higher accuracy for instance to augment the various higher RoF weapons.

Helmets are protective. Helmets are efficient. Helmets are cool. 

I would love a seperate equip slot for helmets.

Even Azrael Supreme Grandmaster of the Dark Angels Space Marines Chapter has no helm eqipped but is accompanied by by a watcher in the dark model carrying his wargear the lion helm, I actually bought 2 of these models to modify and have one with the lion helm equipped.