I like people like that too, but surely the world and more narrowly the human experience also benefit from having people that are competitive or even disagreeable?

Mostly, yes, but I think at its core, the world benefits from honesty more than out right agreeable- or disagreeableness. We should speak up when we feel the need to agree or disagree, but we shouldn't play devil's advocate for the sake of it.

You can be competitive and supportive of your enemies.