From the man page:
A trailing slash on the source changes this behavior to avoid creating an additional directory level at the destination. You can think of a trailing / on a source as meaning "copy the contents of this directory" as opposed to "copy the directory by name"
It'd be great if you could add a checkbox to the "Add Files" dialog to enable "copy the contents of this directory".
I'd love to have this feature as well.
It'd make those late-night-i'm-not-thinking-quick-enough-to-make-sense-of-this-back_up-arRsync-mess-to-not-screw-it-up-in-the-morning backups, a lot less complex when caffeine levels are running on nil.