You can view the referrers for a particular day though. In the WordPress.com stats chart, simply click on the day on which you wish to see your referrers and they will come up with the detailed stats of that day.
I, too, would find it helpful to get a 30-day summary for Referrers, plus maybe an “all time” summary. If that ever becomes a feature in the future.
Would also be nice to have something similar for the “clicks” (for the pages people click on through my blog site). But if it came down to priorities, personally I’d find the “referrers” summary more helpful.
I’m just putting this here in case the people who run WordPress.com are reading this, so they can put it on their list of features wordpress.com users have requested.