If download movie trailers from BD Live for local viewing, those will get deleted.
If you created login/password to certain movie studio BD Live sites, they are also stored in the folder.
It is safe to delete everything in the BD folder since the data are installed from BD Live enabled movies, data from BD Live sites, trailers if stored on hard drive, etc. Technically, BD Folder is a garbage collector.
That's one design flaw with BD Live using storage. It doesn't have any ability to do garbage clean up. It's up to the user to do garbage cleaning.