One thing that is extremely useful to see at a glance is: does this shift need volunteers?
We only care about regular users attached to events here; that is: if a shift has both recurring and single events (very common), we only want to know if there are no users on the recurring ones. If they are removed from one instance via an absence or whatever, it should not flag. Our shift_summaries view takes care of this for us because it ignores single events in an event series.
If the shift only has a single event, it will look at those users and consider them missing if none exists.