I guess a small thing being overlooked here is that, there are man hours and developement time put into programming and optimizing these added characters. They are also ones that were heavily requested BY THE FANS. People actually WANTED Mewtwo back. Of course, "Nintendo isn't doing it for the fans, they're doing it for easy cash". Perhaps. But hey, it's making people happy isn't it? I certainly enjoy playing as Mewtwo a ton even when I wasn't one of the people clammoring for him to return. I especially like using Disable and following with the "point-at-the-ground" down smash. The impact is so satisfying.
Key word: Satisfying
In the end, the value of DLC is all up to how much one actually enjoys the added content. If you really don't care about it, don't get it. This is in regards to DLC made specifically after a complete game is made, by the way. You are pretty much forced to get it with a game like, mentioned before, Dead Space 3 and such. Which is the wrong way to do it.
In general though, DLC is just tricky. There are people who will think it's reaching directly into our pockets and stealing the extra $20 from us no matter what way you explain it. It's like trying to stick your d**k through a Cheerio.