I think you underestimate Young Link's camping game. Throw a few boomerangs and bombs to get the opponent to retreat then boomerang + bomb yourself. Since Marth can combo better at certain percents it could potentially destroy those opportunities for him. Same with any other character who's better at killing you at specific percentsMarth does not care what percent you are are at. By Y/Link blowing itself up the player puts them self's at an automatic disadvantage which Marth basically gets a free shot at you. Then, he can simply proceed to juggle your stock until you die and you are left hoping your opponent is bad enough to mess up for you to get back down.
I'd like to hear what PP has to say about this