Well, X11's selection mechanisms doesn't, as you put it, perform an "automatic copy", it is actually separate from the copy-and-paste system, just like drag-and-drop is separate from copy-and-paste on Windows. X11's selection mechanism is a drag-and-drop operation, only that you can let go of the mouse button between selecting what you are going to drag and dropping it in the destination. It's actually significantly more convenient than drag-and-drop and significantly easier to handle. To make drag-and-drop work as well as X11's selection mechanism, you need to add weird hacks like "spring loaded containers".
Well-behaved X11 applications should implement both X11's selection mechanism and copy-and-paste. To convert between selections and clipboards, you can use any working X11 applications that can hold the datatype you are interested in converting, like an X11 text editor or the xclipboard application.