Where, when and what is the word 圆舞 used for?
My guess, and I think this is a reasonable good guess is that 华尔兹 = "waltz" and 圆舞 = "round dance" and in English a waltz is a type of round dance.

So you don't hear people calling "华尔兹" "圆舞" because "华尔兹" is a type of "圆舞" but 圆舞曲 can be used for any kind of "圆舞" not just for waltzes.

If you use Google to search for the definition of round dance you get the following:


  round dance
  
  Noun
  
  
  A folk dance in which the dancers form one large circle.
  A ballroom dance such as a waltz or polka in which couples move in circles around the 
  ballroom.
  


Also from the wikipedia article on Round dance:


  Modern social round dancing is choreographed and cued ballroom dancing
  that progresses in a circular pattern, counter-clockwise around the
  dance floor. The two major categories of ballroom rhythm found in
  round dancing are the smooth or international rhythms, such as foxtrot
  and waltz, and the Latin rhythms, such as cha-cha and rumba.