Have you ever noticed the way bending the thumb causes the muscles in the whole hand to become ever-so-slightly more rigid? This rigidity is highly useful for a violinist's or violist's bow hand, because a strong and gentle bend encourages the other fingers to do the same, giving the bow hold some structure and support. The left hand is a different story. If the thumb bends a little too much on the left hand, the resulting small amount of rigidity can get in the way of achieving fluid efficiency, and can lead to cramped fingers and poor intonation.
Have you ever noticed that the thumb is kind of the "leader of the pack" as far as the fingers of the hand go? Have you ever noticed how much mobility the thumb has compared to the other fingers? That means it can get into more trouble if it isn't carefully monitored! Where is thumbkin?