Taunting after a KO enrages some folks.
Most folks play more poorly enraged.
It serves to reason that this improves my win rate, which limited trials have shown.
Taunting on KO, and whenever contributes to victory as long as your taunts do not expose you to attack. Would you not use an effective KO move that helps you win, because it's annoying? Melee was all about the metagame, and everyone I ever played with did it habitually, because hey, pointz.
And now, instead of points, we get more victories, probably.
I don't think it's discourteous, it's only discourteous if someone finds it offensive, which I find offensive, so their very offense is discourteous to me, which I'm pretty sure balances the scales.... of courtesy.
show me your moves.