It's intended, adding automatically is a background process which the user may not be aware off, so merging into existing records is not a good idea as there's no feedback for where the newly added file ended up.
And it will only handle files not subfolders. For me, the use case is to easily add individual files I get in my email, or from a website without needing to switch to calibre. It's not clear how to interpret a folder full of books. Are all those files for the same book, or different books? In the normal add process, the user tells calibre how to make that interpretation.