His question, in case you've forgotten, was "Why does rogues get proficiency in longswords if they can't use it for sneak attack." I simply provided him with a straightforward answer: if this bothers him, Rogues don't really need that profiecency. Forums are places for discussing and exchanging opinions and arguments and no one is immune to criticism.You, sir, are deeply confused. If you don't want to have an honest discussion and instead spout off your opinions without fear of logic being applied back, then post on a blog rather than a forum.
Your claim was that because the longsword doesn't interact with rogue features, it could just be taken off their list and "Problem solved." Darts don't interact with monk features, so why should they be on their list of proficiencies? It is a legitimate criticism of your argument and pertinent to the discussion.