At least papers don't have subtitles (I think). When I have to record the title of a book, if there's a colon or a dash in it then I find it hard to know whether it's being used to separate the "title" from the "subtitle". Alternatively, if I know there's a subtitle then I don't know what to do when entering the data into a form that doesn't accept a subtitle. See https://en.wikipedia.org/wiki/Subtitle_(titling) for more information on this point of pedantry.