Here are some of my reasons as someone who has been supporting the character for over two years now:
My favorite song in Kid Icarus: Uprising is
Dark Pit's Theme (which was featured in the Smash Direct btw). I'm hoping a variation of it will play whenever he wins a battle.
Dark Pit isn't just a "bad pit," he's literally a neutral party for most of the game, and is one of the few characters in KI:U that thinks independently. Continuing with Dark Pit's role in KI:U, which also happens to be my favorite 3DS game, he has a large arc that spans most of the game and is given extremely important moments in the story...I won't state them to avoid spoilers, so go buy the game.
Dark Pit is also quite unique amongst the
doppelgänger trope that is prevalent throughout Nintendo's video game catalog: he isn't a one off moment like Dark Link or the other Dark Nintendo characters, but happens to literally carve out his own place in the main plot. I think Sakurai really pushes this idea of Dark Pit being special by introducing
another kind of doppelgänger for Pit later on in the game.
Stepping outside of Nintendo specific tradition and tropes, Fighting games also have a tradition of introducing hidden "evil" versions of characters that are technically clones, but have their own feel and personality. We've seen this in Street Fighter and even in Tekken, who has developers working on this game might I add, so the "evil"/"dark" addition of Dark Pit wouldn't be any different than what gamers have seen prior—well, except for the fact that this character actually has a back story and isn't just a random inclusion.
One thing that should really be acknowledged at this point is Sakurai's clear and present bias towards Kid Icarus: Uprising. I'm all for it. Sakurai tweaked Pit's move set to include weapons featured in Uprising. Palutena herself also uses a lot of Uprising's items along with her heavenly powers. However, all of them are not yet featured—most curiously,
Staffs. I find this curious because there is one staff in particular which would be perfect for Dark Pit...the
Dark Pit Staff. As I have said before, I am a fan of Kid Icarus: Uprising, and would love to see as many of the weapons/items featured in that game to shift over to Smash. Since Pit clearly can not use all of these weapons. One of the main augments against Dark Pit is that he's no different from Pit in Kid Icarus: Uprising, which, ironically enough, makes him the perfect candidate for these unused weapons.
My original reason for liking Dark Pit and wanting him in Smash is really simple, though: I like his character design.