Irrational behavior doesn't equal random behavior and is typically easier to model than actually rational behavior.
Not necessarily. Think about a game-theory scenario, e.g. a negotiation. A fully rational counterparty is relatively easy to model and game. A somewhat insane counterparty is much harder to deal with. That's why signaling irrationality is a common negotiating tactic.
Hence the 'typically'.
I'd disagree with "a fully rational counterparty is relatively easy to model and game" on the grounds that you're basically saying "you can easily win against fully rational counterparties", which isn't the accepted usage for "rational" on LW.
Typically I encounter the "irrational-and-predictable" variant more often than the "irrational-and-unpredictable" kind. It's the actual rational actors that have the oomph to wrap their desires in an enigma, if it serves their purposes (as you say).
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.