I think that up/downvotes being symmetric is a bit illusory. I and many use upvotes to signal “I agree with that/that is a great point, but rehashing the same agreement would be noisy/unneeded”. (And if you have new points, then, of course, comment.) On the other hand, to have constructive discussion, disagreement should be expressed with words, because just the downvote won’t communicate your message. If you then have the same criticism as someone else, you upvote their criticism – I don’t think downvotes fit in here!
On the other hand, I understand the want to downvote. If I see a proposal that I think is a bad idea, I feel the need to boost the signal against that proposal, just to be even a bit more reassured that it won’t get accepted. Most of the proposals that end up downvoted haven’t needed any signal boost, though, so it seems more like a fear reaction.