I've received downvotes I've disagree with in the past but not on the scale @krnel has. The most interesting thing I've read on the subject is this post which was resteemed by @dantheman.
Those who have more risk more. When large stake holders downvote their downvote are remarked and make huge waves in the community even when nobody dares to address them. Those downvoters risk losing support if the community doesn't agree with their downvotes. This is all detailed in the post. I don't know if downvotes are truly a necessity. It's something to keep in mind though.
Excerpt
One More Thing: What about Power Imbalance?
Some people have pistols while others have nukes. The short answer is that those with nukes fear being nuked by others with nukes as well as an uprising of the masses. Anyone who gets persecuted by a nuclear power will find sympathy from the other powers who see the entire economy suffer as a result. Those with nukes (large steem power balances) have the most to lose if social oppression kills the Steem economy.
So should we eliminate down votes? No. We need to carefully consider adding new features to further counter-balance voters who have nothing to lose. Only then will we be able to put a stop to reckless down voting by whales and other users.