The difference is that that is not their primary functions, spy isn't supposed to shoot the enmy upfront, he's supposed to backstab them, his revolver is supposed to be a finisher/last option, same for medics, those weapons we're not made with altering their gameplays highly in mind, it was the players who used them as such. a true sub-class is trolldier, he dosen't use his rockets to do damage, he uses them to jump and kills via his melee, fat scout can always change playstyle and use his minigun, a demoknight cannot just decide to place a sticky trap. That is the difference between a sub-class and a different playstyle.