<p>This ground has slready been covered.</p>

<ul>
<li><a href="http://meta.stackexchange.com/questions/135/encouraging-people-to-explain-down-votes">http://meta.stackexchange.com/questions/135/encouraging-people-to-explain-down-votes</a></li>
<li><a href="http://meta.stackexchange.com/questions/6521/leaving-an-anonymous-comment-when-voting">http://meta.stackexchange.com/questions/6521/leaving-an-anonymous-comment-when-voting</a></li>
</ul>

<p>It will not help.  Basically, people would just post comments that are worthless.</p>