Sometimes it is unnecessary, I agree, and I generally don't vote an answer down, even if I disagree with it.
However, sometimes the answers that people give are rude and unhelpful and sometimes they encourage the person to deal with a problem in a way that is likely to cause them harm. People sometimes give bad advice. I have seen these sorts of answers pop up every once in a while. It's more common here than it should be. Other times people spam the boards or use the Q&A forum for things that this site is not intended to be used.
But I agree with you to an extent. People who are genuinely trying to help and giving the best advice they can shouldn't be voted down simply because someone else is of a different opinion, because a suggestion voted down can end up being the most helpful one but there are people here who are rude and hurtful to the people asking for help and making light of their problems and I think we should have the ability to vote those kinds of responses down to show that it's in bad taste or bad advice. The privilege should not be abused, but should also not be taken away.