Most of the traditional fiddle players who I know who either started out classically trained are very pleased for having the training. I also know several fine fiddle players who started out learning by ear, and later took up classical violin to help with thier technique. All were very happy they had done so. One of the biggest advantages of classical training is that it teaches you to play in any finger position, and this is of great benefit with any kind of fiddle music. Itzak Perlman seems to do a splendid job with Klezmer fiddle. It's easier to play folk music if you start with some classical training, and although it's great to be able to play by ear, it's invaluable to know how to read music competently.

With regard to Bach's double and triple stops, he wrote for the Baroque violin which had much flatter bridges than the classical violins used today.