Suppose:

 - You ask a question;
 - Someone (well meaning though he may be) writes an answer which simply does not answer your question, but rather tells you what you should and shouldn't be doing;
 - The answer gets a significant (> 5) number of votes, and some positive comments.

Now when people look at the question, they see a high-vote answer and think the problem is solved. What can one do, other than a comment saying "that doesn't help me at all"? I can't downvote by more than 1, plus, that won't help since it's the only answer anyway, plus, downvoting is expensive.