Today,  I attended a "software training course" -- or, well, I watched
  it from a safe distance without having  to  participate.  An  "expert"
  taught  the  attendees  how to use a specific program. The funny thing
  about this: That program had a (quite simple) GUI.

  Most people argue that CLI programs are hard to  learn  and  that  you
  don't need to learn how to use a GUI program.

  Well,  yes, you *do* need to learn how to use GUIs. The mere existence
  of that software training course proves that. You just need  to  learn
  *different* things when using GUI programs.

  The question is: Which is easier to learn and which is more powerful?