Well for more Reason than one. At First: For me personally Persona 5 is one of the best JRPGs of the century, if not of all time... and Joker is one of the coolest Characters out there... I'm normally more the Detective Type, but this (Gentlemen)-Thief was amazing.
Secondly: I get into this Series with Melee without knowing most of the IP's Nintendo offered. Until i played Melee i never new about Zelda, Metroid and such.. except from Pokemon, Kirby and Wario at the Gameboy i was pretty fresh, so for me in my Perspective, Super Smash Bros isn't only a great Crossover for People who know and love Nintendo, it's also a perfect Game for Newcomers which never get into the touch of this Games and than have the chance to meet this Legendary Characters and get them interested in their Main-Games. So to acknowledge (maybe for marketing-reason, maybe for truly honoring the series - idk what the reason for Nintendo is) Persona 5 and honor it by giving us Characters, Stage and Music (and the Score is amazing), to see that some even might get interested in this Game, excites me... and as much as i'd love to see another Zelda-Character and still hope for Rex & Pyra... if they Honor with the DLC's Characters like this, i'm all up for it!