There's no reason for one, because in the end it will just be abused as the like button is. Plus you just explained why one isn't needed because as you said, you can reply and say "I disagree" and say why.
The like button serves no purpose because in the end you can't tell if someone actually likes what you're saying or if it's just buddies on here that automatically like what certain posters say.
I would do away with the like button, but if it's staying, I think the ability to undo your like needs to be added. I've accidently liked posts before that I didn't actually like.