Ironically of course, headgear and gloves are known to be the reason more boxers are affected by long term problems than people who bare knuckle spar.
You also wouldn't hit someone in the head with your fist if you know what you are doing because you would probably shatter a knuckle, hence, historically, bare-knuckle bouts were about slowing and draining your opponents energy until they couldn't get up from heavy body blows than knockouts.
It allows you to absorb a larger number of heavy blows to the head without showing any outward damage. There is an argument that the risk of brain damage is therefor greater because the bouts go on longer. I personally haven't read any studies on the matter so I can't comment on the medical validity. In practice I prefer to fight without headgear. This is completely subjective but I feel sharper afterwards, whereas 6-8 rounds with headgear leave me foggy for a day or two.
It is interesting. Technically it should be the same amount of energy transferred but without localised damage (eg, concentration of force through a knuckle into an area of bone or tissue). The head will still be jarred, so your point is pretty solid.
Regardless, though, in this specific example - if one of these guys throws a stray fist to the mouth or jaw (not so much for body shots because they're not exactly doing much of a job of it), its likely to break something, and if they are boxing in a training only setting or socially they're not going to be taking enough blows for concussion / brain damage to be a real issue.
You also wouldn't hit someone in the head with your fist if you know what you are doing because you would probably shatter a knuckle, hence, historically, bare-knuckle bouts were about slowing and draining your opponents energy until they couldn't get up from heavy body blows than knockouts.