aBoo3
It is indeed useless to have more than 3 copies of a bronze unit -- except havíng 3 of both "normal" and premium versions if you'd like to eventually have a 100 % full collection like many players do.
I'm guessing it's not included in the OP because the deck builder screen in-game used to tell you the maximum number of copies you could have (3x Bronze, 1x Silver and Gold). With the new UI that little detail is no longer present.
As for Bronzes that never see play, I daresay at least a few players are using all of the cards. Even competitive players sometimes make uncommon techs to counter common meta decks. For example right now you might not see Mushrooms or Dimeritium Shackles in any top-rated decks, but that doesn't mean someone tired of Nekker Consume or Axemen doesn't tech them in.
In any case, every time the game gets a new patch the meta also changes, affecting the play-rates of many cards. What is never used in the current patch might become popular once the next update arrives.
The opening post is out of date in various ways (starter decks, some cards abilities and some game mechanics), having been last updated in July.
Currently
this thread is likely a better place for questions, if you want up-to-date information or quick replies.