Yeah, we have to design for two different factors here.
In a battle, it's important to protect your head. Therefore, wearing protective head gear is a thing most soldiers are rather interested in doing, across centuries and cultures.
...And on film, protective head gear tends to block out at least part of the actor's face, and is thus not desirable. Most directors find an excuse for actors to lift visors, remove helmets, and run around without the pesky headgear in the way. Even if it's a battle scene and really not a smart/recommended action.
There are exceptions. Sometimes, seeing the actor's face isn't important. There are roles where it's essential the mask stay
on, and sometimes the mask in question is a helmet.
So, in V-for-Vendetta, it's not like you see Hugo Weaving's face. Instead, you see the smiling Guy Fawkes mask.
https://i.ytimg.com/vi/KKvvOFIHs4k/maxresdefault.jpg
And in Monty Python's Holy Grail, the Black Knight is able to get through his entire skit with his helmet firmly in place. We don't need to see the face behind this helmet, apparently:
No one complains that the Witch-king keeps his helmet/mask in place throughout the Lord of the Rings films...there is no 'face' behind the mask anyway!
Another way to deal with this is to let a character wear a full face covering helmet....briefly. So, we do see it on screen, in a 'see, they aren't an idiot wading into this battle with a bare head!' but...that helmet isn't going to be there when you get to a scene with any emotion in it.
Frodo and Sam disguise themselves as orcs. They wear these costumes for, what, 3 minutes of screen time, if that? Just long enough to make use of the disguise. Then it's back to hobbit-clothes for them.
http://corecanvas.s3.amazonaws.com/theonering-0188db0e/gallery/original/frodo-sam-rotk.jpg
Peter Pevensie wisely lowers his visor before charging into battle (bareback on a unicorn, but that's another conversation). And yet, the entire helmet is gone by the time he faces the White Witch. A similar thing happens during the duel with Miraz in Prince Caspian.
https://i.pinimg.com/originals/f7/38/a4/f738a44fcd6f989342560a2216393340.jpg
(I haven't seen any versions of
The Man in the Iron Mask, so other than knowing that there is indeed a metal mask over someone's face, I have no idea how the situation was handled on film; I presume you see his face at some point?)
If we give the dragon helm a movable faceplate, that allows actors to wear the helm on screen, and lower the visor for an instant in a scene where we want said actor to look menacing and invincible. But then to lift it to deliver lines, have facial expressions, etc. It's probably beneficial to make this a movable piece. And since we know there's a scene where Glaurung taunts Túrin to look at him....we'd want the option to have Túrin flip up the visor on the dragon helm if he is indeed wearing it at that time.
I'm now imagining a helm with a visor that lifts, but with a split down the middle, so it can slide back along the base of the crest. Not sure how practical that would be....
But then the quandary is, how do you have a piece that slides into and out of position over the face...AND also have a dragon crest? Because either the flipped-up visor obscures the dragon and makes it look silly, or else the dragon is in the way of whatever moving parts you want to have. It's a more difficult design to make look cool and not clunky, that's for sure!