Wrong answers can add value because:

• The person who posts the wrong answer will almost always be corrected by someone who knows better, and hence will learn from it
• People who thought the wrong answer was right, and just another way of approaching the problem, will see the correction and realise that it was wrong. They also will learn from it.
• Sometimes an original post will not make the question clear. Occasionally an answer will be posted that others disagree with, and the OP will later state that the 'wrong' answer correctly interpreted the question.
• Not infrequently a wrong answer will cause the OP to clarify their original post, leading to better answers.
• People who post rude and personal responses to a 'wrong' reply will often get downvoted, and perhaps learn to deal more gently with people.

This is not a commercial support site, its a community forum. Everyone has something to contribute, everyone has something to learn.

Update: inserted the bold word 'can', in response to PerlMouse.

Just to clarify, if the OP gets 2 right answers, 2 wrong answers, and those wrong answers get highlighted, then at least 3 people have learned something. How is that bad?