I'm suggesting these privileges because comments are one of the few things that elected and appointed diamond moderators can do something about that the rest of the community cannot. Other than flagging, there's no way for regular users to fix problems with the comments on a post. Having a powerful privilege to aspire to provides a better incentive than providing somewhat less-powerful privileges. Anecdotally, people most look forward to the 10k privileges which introduces the final piece of community moderation: post deletion.
These are exceptional users, so they participate (as a group) far more than others. If we look at users with more than 5 reputation, the median first post after getting participate on meta is 215 hours. Compared to their later behavior, trusted users start slow. Notice that they also slow down again after hitting 10k or so. Critically, the slow down after 10k also occurs for 30k users (N=429), so it’s not just an artifact of the cutoff.
There’s circumstantial evidence that increasing the top privilege level increased participation at higher levels. More to the point, the relatively underpowered 15k and 20k levels appear to be less effective at motivating users than 10k powers. So introducing something really helpful, such as comment moderation, seems in order.
However, I think comment moderation is mostly a waste of time. I'd much rather let the system handle comments. Unfortunately, there are already so many comments that should have been edits (or even answers) instead that we can't blindly hide/delete old comments. Creating this privilege will mean committing to manual comment moderation for the foreseeable future. Comment moderation is tedious and unrewarding so I'd rather have our top users do something more productive with their time.