Well, if it is obvious that it is a bad answer, it will be obvious, so we don’t need to have a ‘bad answer’ option to point it out. If it is a case of misinformation or misunderstanding the question, than if someone feels compelled to point that out, they can do so with a polite comment. If it is abusive, flag it. I realize I haven’t said anything others haven’t.
However, I’d also like to point out that I can see someone getting a ‘bad answer’ tag and then the rest of the question page would be filled up with things like that person or others debating whether it really was a bad answer or not. People wanting to know who tagged it as a bad answer, hurt feelings, etc. and discussion of the original question will be lost.