I've always rationalized the need for banks in that it would be unseemly for nobles to pass currency to each other like commoners buying bread in the square. Nobles use official bank couriers when dealing with each other. For all the paraphernalia our characters buy with gold, they buy it from peasants, so the fact that they're paying at all is more like charity which is totally fine, if not beneficial, for other people to see. I'm sure you can poke holes in that reasoning, but it keeps my happy little BM-world afloat.
