I think the rule is mostly for dueling -- back when dueling was a thing in SF. (Though it is an issue for brawling as well.)
Any blade weapon skill can be applied as -DM if you're fighting someone using a blade weapon against you. So if you have two people who know how to sword fight they will each have penalties as they roll to hit from their opponents parring skill. This extends contests... which means the Endurance rules might come into play. (And, again, Brawling provides a similar Defensive penalty on the attacker if the defender as a Defensive expertise in Brawling.)
Most people play Traveller in the uniformly high-tech setting of the Third Imperium. But the rules are there to handle worlds like those found in Jack Vance's Planet of Adventure series, the fisticuff action of The Demon Princes books, or even a planetary romance setting like A Princess of Mars. In those tales the the protagonists often end up without firearms -- either because they run out of ammo, their weapons have been taken, or their are not many firearms available.
Any specific setting detail will obviously impact how many, if any, Blows or Swings are regularly made in a game session.