With a minimum length set. So we can see whether the reasoning is good or not. Sometimes people are reactionary, don't digest what is being proposed properly, or misunderstand what is being proposed and cast their vote on that basis (take this RFE of mine as one example)--this proposition would expose their poor reasoning, plus some people feel snubbed if their idea gets rejected without anyone displaying their reasoning; whether that's rational or not it would still stop feelings from getting hurt (someone over-reacted recently and asked for their account to be banned after such a rejection--their request was honored--while he chose to respond poorly I'd rather avoid such things as much as possible). Plus, who knows--some people might genuinely be trolling the votes--let's see how many rejections without explanation this one gets. :)

EDIT: And display in the vote-casters message-header which way they voted--if the vote wasn't sincere and earnest enough that they would stamp their name on it and stand up for it, then perhaps they don't feel strongly enough about it that they should be casting any votes.