Okay, so let's say that they we're just "running into it" as you claim. Watch the video again. Watch the VERY FIRST clip of that video again. Did Marth have any other options in the moment before he was struck? Nope. The only option was to use his up-special, and the player was likely not expecting it, since Luigi's taunt is rarely used.
I think that from this conversation, as well as the video (thanks
Filosafer!), I've concluded that Luigi's taunt, while extremely situational and extremely tricky to pull off, Luigi's down taunt allows him to pull off a deadly meteor smash without having to leave the ground, and getting a KO with the moves is immensely satisfying and gives off a pizaz that few other moves can.
Also, who the heck do you think you are?? Acting all mighty, calling yourself smart, and other insulting the intelligence of other players. You say that no pro would ever get hit by the taunt, but how do you know that the players in that video weren't pros? Do you honestly think that you'd never be able to hit a "pro" with the down taunt, even if the person playing Luigi was crazy good? Do you think of yourself as a pro? Do you honestly think that you could go toe-to-toe with some of the best Luigi players in the world, hell, even the Luigi player in that video, and actually expect to never once get hit by that taunt. I mean, according to you, you're smart and only "dumb" people would ever get hit by it, right?
You seriously need to re-evaluate your tone, your attitude, and choice your choice of words. You've barely been on the forum for a month! What gives you the right to insult other people's intelligence? What gives you the right to say that you're better than others? You have no credibility, and your arguments are backed by zero evidence. Using insulting language and being conceited doesn't make you anymore likable buddy.