I think it depends.
For awhile, before I set my profile to private (mainly because of how many weirdos were on it), I had a disgruntled girl who didn't like my answer to her question and who had about 9 fake accounts go around disliking my answers with all of them. The most humorous part was when she used all of them to reply "I agree" or "Couldn't have said it better" to her rude comments on my opinions. Hmm... I don't think it's a coincidence that I kept getting 9 down votes, there were 9 accounts bashing me who all had similar names and were either 29 or 30, and all had the same typing style.
Some people simply down vote out of a difference of opinion, even if it's something small.
Then there are people who down vote because the opinion is rude, unhelpful, or gives bad advice. This is what I use it for.