Wouldn't they shrink?
Seriously, I'm absolutely staggered and surprised to learn that North Sea pilots don't already wear helmets. Here's one bean counter with whom you wouldn't have any argument!
Apart from cost, they only thing that I can think of that
may be a disadvantage is whether a helmet would be an encumbrance in the event of a ditching. I've no idea whether they would, it's just a thought and a question!?
Cheers
Whirls