The idea that Dark Pit is somehow a 'bonus' or 'free' character that expended next to no development time is demented. He's appeared in a CG trailer, has his own voice acting, his own balancing requirements and even his own victory theme. Even as a clone he is directly sapping development time that could have been spent on a clone not from a series that's being massively overrepresented.
Let's not go flinging hyperbolic statements around, now. We're perfectly capable of having a rational and polite discussion without throwing out phrases like "demented" and "sapping development time". I get that you don't like him, but come on. Be respectful to the fans.
Now, that being said, what difference does it make that he appeared in the trailer and he has his own voice acting? We don't even know if he was going to be playable at the time the trailer was animated or if Sakurai just threw him in there as a placeholder for whatever role he would have in the game. He was obviously going to be in the game overall, and if he was a playable character at the time of the trailer's reveal, I think Sakurai would have announced him there and then. Possibly he just realised he had some dev time left to create a new character, saw Dark Pit and realised there's already a precedent there. People were, for better or for worse, already expecting him to appear, so it's not like it should come as an almighty shock that ol' Pittoo is in the final roster. And it wasn't even his own trailer - he just made a cute little cameo at the end that some people rather appreciated for what it is.
Also Pit and ol' Pittoo share the same voice actor, so it's not like it's a huge deal to ask him to record some new lines and throw in some grunts and growls for good measure.
I don't really get the argument that "well he could have created another clone instead". I'm sorry, but I don't think it's as easy as all that. You're very welcome to rebut my sentiments if you so choose, but I can't quantifiably conceive of an alternate clone that would appear in his stead. Out of the potential candidates, there are mitigating circumstances there: Dark Samus was already programmed as an Assist Trophy, so to include her would be to undo the time and effort they'd already put in; Alph is arguably too complex and would be tricky to implement on a tight budget and with the clock running out; including someone like Dixie Kong or Funky Kong, as I say, may be taken (arguably unjustly) as a slap in the face to the fans who wanted "real" characters and could very, very easily lead to a future "Captain Falcondorf" situation in that respect; Mewtwo could theoretically take his place, but that would mean cloning him directly from Lucario, and we all know the kind of backlash that that would result in. I wholeheartedly think Sakurai, under pressure and hurrying to finish the game in time, just said, "Right, screw it, we'll throw this guy in. I know how he works, we've already got his model and his moves in place, and maybe the fans will like it."
I'm really, really sorry to come across as such an arrogant snob here, but I think we're all being incredibly hard on Sakurai. He's gone out of his way to create this beautiful game and has developed health issues as a result; he doesn't have to do any of that, but he does, and we're just going to throw his generosity back in his face because it's not what we wanted? Why reject free gifts for the sake of making our points? I get that we're upset and that we want to vindicate our arguments by responding as such, and that's fine. It's a natural human response to counter anger and wrath with, well, anger and wrath. Vent your feeling all you want, but please, for your own sake as well as ours, just try to look at things from other perspectives, too. Criticising Sakurai and using harsh turns of phrase against his choices is fair game, but I'd urge people that are frustrated by the whole issues therein to just step away and focus on the positive side of things.
At the end of the day, we don't really know what Sakurai does, because we don't work in his office and we don't do what the dev team does. He's the director of the series and that comes with certain responsibilities - he has to make the tough calls, and we have to live with them. They may not be perfect, or maybe not even good, but they're the choices that they are. Let's just all be cool and have a good time with the game that we have.
Of course, I'm not going to convince people whom are already firm in their beliefs, nor do I intend to. And that's fine. Just like everybody else, I want to vent my own feelings and frustrations and just look instead at the good things in the game. Each to their own, my friend, each to their own.