I use compound clips to group "things" that I am "done with". For example I was on a gig yesterday where the client wanted to use just certain parts of a music bed. I cut the song up to make it work, did the mix and was finished. For simple organization, I put all the music cuts into one compound clip. Same thing with various sound effects. Of course FCPX then creates a new clip in the event for each.
Secondary story lines for me are a way to keep my options open when cutting cover footage and the like. I probably don't use them as much as I should and need to learn more about using them.
Senior Editor/Post Production Supervisor
Professor, VCU Brandcenter