Indeed. But mine is more..... constructive : I said what could be the advantages for the other classes, and the guy who made the thread that you linked just said "Give the FB to all trololol" (I don't want to insult anyone, I'm just saying that this suggestion wasn't justified). Of course, people could say "If we give the FB to those classes, why we don't give the GRU to the Scout ?" and coul say this with every weapons of TF2 and the answer would be the same : it would transform the vlasses too much, or not interesting for the classe itself ! Here, the FB is a shotgun, the Soldier, the Pyro and the Engineer got a shotgun as a stock weapon, and this weapon, compared to the Reserve shooter, isn't situational and every classes can obtain the bonus and is affected by the downside of this weapon.
Note : I say I was the first to make this thread because I didn't found any thread talking about it when I've done a research.... Strange.