With Marth, it becomes very dangerous for his opponents to be above him. They will be forced to go to the ledges to get back on the main stage if they make a mistake. If you've ever seen Sethlon play Roy in Project M, that's the kinda stuff you want to achieve. Make everywhere above you a hugely dangerous place to be. Try your damnedest to keep them in the air with up air. It overall makes his nerfed air game force to be reckoned with once again.
Marth can also use up-b from center stage in basically instantly land on the middle platform. This allows for some Dancing Blade into up-b combos that are completely safe, and you get the extra hit from up-b in.
This is moreso speaking about Battlefield, though.