It's optional to hide the helmet.
Don't worry, you won't worry about your own helmet disappearing

It's an option for all the players who like to play without helmets but don't want to lose the AWESOME bonuses that come from wearing a full set.
It makes no sense at all.
"I'm not wearing a piece of safety device, but I still want the benefit".
Just boost smithing, use the Lord Stone, enchant a circlet or other pieces of equipment, etc. There are many ways to replace a removed helmet other than it being "hidden". If you don't want to wear a helmet, fine, but deal with the consequences.
Why not have a "hidden" sword while we're at it? I'm swinging nothing at my ennemies yet they take damage. Same logic. And in both cases it's ridiculous.