With regards to power armor, my understanding was that the average CSM does not remove their power armor - it's fused on. This was in the early fluff, where horns and spikes were more common, and Daemon Armor and Spiky Bits were upgrades you could take.
With regards to the psychology of World Eaters leaders, I think about that from time to time. It has to be similar to living as a functional alcoholic, they adapt to life with this incessant urge rattling around in their heads. I imagine the average Berzerker would spend time seeking balance between the demands of the Nails and his own self-preservation, knowing he can't fight *all* the time yet being driven to do so by the slightest stimuli. Some new normal would emerge, where his primary drive becomes channeling those urges in a satisfying manner while avoiding the situations that would set him down a path to his own destruction (i.e. killing everyone in his warband, since there's an undeniable safety in numbers.)
Leaders have to be constructive, they have to set a goal and get others to follow. Being in charge of a Berzerker warband would be like leading a confederacy of murderers. With the constant risk of enemies within and without, rationalizing this position would be impossible without an arrogant belief in his own superiority and the promise of some reward beyond measure. Add to that having to deal with the Butcher's Nails himself - they are always on - the day-to-day existence of a Berzerker leader becomes a little more narrow. He has to command fear to the point where his men aren't going to kill him, he has to satisfy his own urges with a constant stream of violence, and he has a powerful incentive to earn the favor of the Gods in the most efficient manner possible - otherwise he becomes just another skull on the throne. He surrounded by others with the same drives and knows he can be replaced easily.
When I think about it that way, I wonder if a Berzerker could go around without armor, even for a few minutes. It feels like that would be seen as weakness and result in a quick decapitation.