Yes, there really are good reasons for holding the violin and bow the "correct" way. The purpose is to get a good quality of sound without trying to work against your own body. I make a point of telling my students why they should position their shoulders, chin, writsts, fingers the "correct" way. For example, it's easier to get a good tone when you place the bow midway between the bridge and fingerboard, with all hairs touching the string, and move the bow parallel to the bridge. It's easier to move the bow this way if you hold your violin in a plane roughly parallel to the floor. If you grip the bow tightly, like a vise, you'll lose control and get a scratchy sound. If you hold the fingers of your left hand over the strings, it will be easier to play the notes in tune. However, if someone sounds good but doesn't look like the photos at the beginning of the Suzuki book, there is probably no reason to change.

One of my current students started teaching himself fiddle. He grew up listening to his family members playing fiddle tunes. After playing for a few months, his mother told him to take some lessons. :-) When he started taking lessons from me, his playing deteriorated for a short time while he learned to realign his body. Now he sounds great, even to his mother. He is very talented, and I'm so glad I've been able to help him make the most of his innate musicianship.