When I create a new document aimed at containing only plain text, I am not obliged by Ubuntu to add a .txt extension to its name. It works indeed very well: gedit opens it without problem, understanding very well that it is only text.
The only two pro arguments I have found from now on for adding an extension are 1/ interoperability with Windows systems and 2/ avoiding confusion with folders having the same name. Nevertheless those two arguments do not convince me at all. As a consequence, should I keep the reflex of adding an extension to files or not?