I agree that that particular reason doesn't really hold up to scrutiny, but I disagree that all that remains is a de facto witchhunt and personal drama celebrated in a public space. The kind of behavior ialdabaoth is calling out can exact a toll on the community beyond just messing up the karma signal. It suggests a kind of passive-aggressive hostility that a lot of people find very unpleasant, unpleasant enough that they might think it not worth their while to be part of a group where they have to deal with it. When you're a participant in a community, and someone within it is behaving like a jerk in a manner that could drive away valuable contributors, I think it's a good idea to call out said jerk (assuming private requests to stop the jerky behavior fail).
It is perhaps worth remembering that the original stated purpose of downvotes was to allow LW users to weed out low-quality contributions/contributors in an egalitarian fashion (that is, without the need for privileged users to perform privileged acts of weeding).
Consider an egalitarian mechanism X that allows a community to keep out low-quality contributors. The only way I can think of for such a community to prevent a rogue agent A from using X to keep out high-quality contributors is to ensure that the bulk of the community can tell the difference, ag...
I'm just tired of the signal pollution, and would like to be able to use karma to honestly appraise the worth of my articles and posts, without seeing 80% of my downvotes come in chunks that correspond precisely to how many posts I've made since the last massive downvote spree.
EDIT to add data points:
Spurious downvoting stopped soon after I named a particular individual (not ALL downvoting stopped, but the downvotes I got all seemed on-the-level.)
One block of potentially spurious downvoting occurred approximately one week ago, but then karma patterns returned to expected levels. I consider this block dubious, because it reasonably matches what I'd expect to see if someone noticed several of my posts together and disagreed with all of them, and did not match the usual pattern of starting with the earliest or latest post that I had made and downvoting everything (it downvoted all posts in a few threads, but not in other threads), so I'm just adding for completeness.
Spurious, indiscriminate downvoting started up again approximately half an hour ago on Sunday (12/1/2013), around noon MDT.
Edit: And now on Tuesday, 12/3/2013, at 10 AM, I'm watching my karma go down again... about 30 points so far.
Edit: And now on Saturday, 12/14/2013, at 2 PM, I'm watching my karma go down again... about 15 points so far, at a rate of about 1-2 points per second.