I agree with your assumptions, and I've deleted that directory many times before with no ill effects.
I've done it multiple times without issue. FMS just recreates it if it needs it. It's not there in a fresh install, so I'm pretty sure it follows the same standard as backup folders (once created, free game).
I've never seen anything in the documentation to suggest otherwise either.
Thanks Mike and Chris.
It's for backup/security reasons
If one deletes some files from within the admin console, there is no 'undo', no 'last backup' - therefore, FileMaker Server will not delete the files immediately but moves them into that folder.
One can delete them manually, no problems. IMHO, that's the only way to make sure that nothing is deleted without one last (admin-) user-action (-: