Does a story require both a well-fleshed out villain and hero?

Two things come to mind:

1. If the story falls within a genre that requires the hero to be not only a main character, but an essential one, then, yes, the story requires a well-fleshed out hero, simply because this particular kind of story does. Think romance. Without a well-fleshed out hero, the story would fall flat no matter how interesting the remaining characters were.
2. In other types of stories, if you have an interesting villain and not a well-fleshed out hero, then you have simply turned your villain into the hero and the ‘hero’ into a minor character. Not all heroes are nice people and having a villain that is also the hero isn’t all that unusual.