There are a few situations where it would be really nice to send a single email instead of multiple ones. Here are a couple examples: 1. I reply to someone's review and update the diff. 2. I reply to two separate reviews on the same request within a short period of time. Putting a slight delay on emails and attempting to group relevant ones would reduce the amount of email traffic back and forth.
Overall, I find reviewboard REALLY verbose. I really want a way to turn down the number of messages it sends to me. Also, review authors should be able to do things like "minor edits" that let them make changes without sending email.