Honestly my post was mostly a joke, but I don't think mandating a rationale is a good idea, and I give a rationale for ~85% of my downvotes. Probably a third of those don't deserve rationale--I'm responding to someone who is pretty overtly trolling or otherwise trying to incent a flamewar. Beyond that, I will occassionally not leave an explanation because
1. I don't have time to type out an appropriately nuanced comment, and an un-nuanced comment is hardly better qualified than a driveby downvote.
2. No matter the quality of my explanatory comment, it will only be a target for partisan downvoters. I don't care much for my karma, but I do care for my time and I don't want to waste it so trolls can give my view an air of illegitimacy.
Besides, while it is annoying to be downvoted anonymously, no one owes me an explanation. It's a courtesy at best, and any attempt to regulate it will be defeated by insincere feedback.