I have been a victim of serial de-voting over last 4 months, which system did recognized couple of times, but it gets bit annoying when someone continuously devotes your question/answers without putting any comments down.
Here are the links to some of the discussion (ad nauseam) that has occurred on meta.stackoverflow.com.
Just for those who aren't aware, SharePoint.StackExchange.com is built on the StackOverflow platform, so we (the SharePoint moderators) aren't actually responsible for the way the site works.
Requiring people to explain their downvote only encourages argument. If allowed, I see this scenario playing out:
It's not required to explain an upvote; why should you explain a downvote? People are already (mildly) disincentivized from giving them in that a downvote removes a little bit of your own reputation as well.