I fully agree. After a download completes, I always have to manually check whether there is something left in the download folder. Then I have to decide whether to delete or to move those remaining files. And finally I have to delete the empty folder. Since everything else is cleaned up so nicely, it would be great to have ABZ fully clean up automatically.
My specific suggestion would be either to add an option "move any remaining files to unrar folder" in the unrar setup. Or, as a more customizable option, let every user specify the types of files he wants to have either deleted or moved to the unrar folder. The types of files that come to my mind are *.txt, *.nfo, and *.jpg. This could be implemented by having a list of the mentioned files (plus the already included *.sfv, *.md5) with three radio buttons next to each file type: i) no action, ii) delete, iii) move to unrar folder.