I didn't use the word "simple" to describe Jecht, I used it to describe Shuyin. Jecht still had emotions. He still cared for Spira...and his son.
Shuyin was the simple one. He had no real motivation, no personality, no character. I don't blame him for that. He was a shade, an empty husk of rage and hatred that had degraded into mindlessness over the course of a thousand years. But it makes him rather boring if you want any character development. And it still doesn't address the issue of having two characters with the exact same moveset.
Oh, and your last paragraph is correct if you think of things that simple. Hero on one side, villain on the other, and they fight. But there is a lot of character development in this game, even though it is a fighting game, and Jecht's character is not just that of a simple villain. Even though he counts as a fighter for Chaos, his feelings for Tidus still shine through, and are integral to the story of Tidus's section of the game. Kefka even claims he is a fighter for Cosmos at one point. That is why Jecht is the perfect villain from this game. He has a close relationship with Tidus. And the relationship is antagonistic. Tidus has hated Jecht for years. But they still love each other and that still shows. It's a great relationship that is being brought out perfectly in Dissidia.
I liked X-2, but Jecht is the best choice of villain from either game given the way Dissidia is being written and designed.