Think about how much effort everyone spends talking about karma, and trying to fix karma, and protect karma from abuse. Is all that effort worth it for the low signal karma provides? Who cares about amassing internet points, let quality speak for itself.
Suppose some comment thread has a thousand comments. Without a karma system (as e.g. on Slate Star Codex), I can either waste several hours reading them all, or quickly scroll the page looking for something catching my eye and hope I don't miss something interesting. With a karma system, I can entrust the readership with the task of telling me which comments are the most worth reading, and read them first.
I've gotten sufficient evidence from support that voiceofra has been doing retributive downvoting. I've banned them without prior notice because I'm not giving them more chances to downvote.
I'm thinking of something like not letting anyone give more than 5 downvotes/week for content which is more than a month old. The numbers and the time period are tentative-- this isn't my ideal rule. This is probably technically possible. However, my impression is that highly specific rules like that are an invitation to gaming the rules.
I would rather just make spiteful down-voting impossible (or maybe make it expensive) rather than trying to find out who's doing it. Admittedly, putting up barriers to downvoting for past comments doesn't solve the problem of people who down-vote everything, but at least people who downvote current material are easier to notice.
Any thoughts about technical solutions to excessive down-voting of past material?