I'd like to suggest the addition of a "catch-up" mode for getting backlog.
When catching up on a show SickBeard could check how many unwatched episodes are present. Once an episode is marked as watched or deleted it should go and fetch enough to ensure that X episodes are always present to be watched. I'd set this value to 2 by default but would make it variable.
If shows are moving out of the retention window the process should download all shows nearing that point.
The watched flag could either be set on the interface or automatically established by scanning the directory - any deleted episodes would then also count as "watched".
The main benefit would be to minimize disk and bandwidth usage and prevent download of entire seasons even if the user loses interest some way through the show. It would also reduce the need to manually retrieve backlog.