Well, if you can delete subfolders, then you aren't deleting the main folder, either (unless it is empty?).  So isn't this really a 'delete_messages' method?  In which case, how does it differ from the 'clear' method?  Only in that it deletes the folder if it is empty?  That doesn't sound like a good API to me.

I find myself really confused as to what your goal is here.