I think it would better if it was required. I see no harm in giving a reason why you have voted a certain way. This should make votes more meaningful and accurate. It should hopefully make people think a bit before they vote as well.
It will be easier to spot where people have been silly, or are voting because of a grudge. If the comment reads "Spams a lot, is rude and doesn't provide accurate answers" you know its probably more valid than "Muh ha ha ha pwned!!"
