Undoing a vote, even if it is useful, would add clutter to the UI. Because I always enjoy the sparseness, but would rarely want to undo a vote, I think the site is better off without the feature.
No, it would just leave the arrows in place except highlighted when "voted". You could then click it again to unvote, or click the opposite arrow to reverse vote. Just like Reddit.
I'm not saying I want unvoting, as I don't really care, but I don't think it clutters the interface in any meaningful way. (Though it does make the pair of arrows multi-state, which could be confusing for some users. Doesn't seem to bother redditors though.)
Adding undo wouldn't add clutter, just see how reddit does it.