the issue with gripping the bow by the metal is that your wrist remains static, you can't pronate your hand as much, this will lead to poor bow control and lack of phrasing especially in bel canto like passages. the purpose of the thumb in the frog groove is so your wrist can be free. I personally use Franco Petrachi's bow hold, because my teacher was a student of his and we all come from the Italian School. I hold the bow with my thumb a bit further back, so it's not in the "groove" but at the crevice (u shaped area) of the ebony frog. I notice this gives you more power, and you can relax your hand more, and let the "natural" weight of your arm ease into the bow hold, instead of tensing up.