Oh don't go opening this old chestnut again, many a long night was spent in the longhouse discussing this prickly issue in the pre internet forum days.
I thought this one was dead and buried with the advent of diesel engined longships.
In pre diesel days the pro lobby argued the need for helmets in order to provide some degree of protection in the event of main halyard failure which of course caused the yard to come crashing down into the boat with little warning. The anti-lobby meantime preached in favour of regular inspection and maintenance/replacement of running rigging in order to prevent such accidents in the first place and pointed out the risks should a helmet wearer fall overboard when the extra helmet weight would cause the helmet wearer to float lower in the water increasing the risk of inhaling saltwater through mouth and nose.
The fate that the Norns have written will not be changed by a piece of leather