This feature is already on our wishlist.
I am adding this here because this post reflects the exact question that I have.
When you say:
If you add a folder to the backup sources, all subfolders will be automatically included.
Do you mean that I cannot just get a newly created folder or file to back up unless I physically choose to mark it? I just want to be sure I am not missing something.
If you create a new subfolder in a folder that is already in the backup sources, the new folder will be backed up without having to manually add it in the backup sources.
If you create a new subfolder in a folder that is not in the backup sources, the new folder won't be backed up until you manually add it in the backup sources list.