Okay, you just nerd sniped me.
Opt-in makes it fairly pointless. But without opt-in, if you want to make it prohibitively difficult to write a script to find a member's complete voting record with reasonable speed and near-total confidence:
I think you would need to make this only available above a certain karma threshold and with a significant time-delay before changes took effect, (so users can't see a page with arbitrary weights on-demand). And only permit a few thresholds of weighting and require several users at each level (a single user with a fractional weight has no privacy).
Then someone can just weight sockpuppets who have no votes, or members who weren't active during the time period you're interested in, or ...
And if there's a way of getting around that, two users collaborating (or one user who's posted a rationality quote with a sockpuppet) can still blow this out of the water.
And all this makes the feature even uglier than it was before.
As to ugly: this inferential gap is probably larger than I care to bridge.
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.