Reducing the number of branches offers the potential for huge savings, since these account for about half of all costs in retail banking. Yet the dilemma facing banks, new and old, is that the most complex and profitable financial products, such as mortgages, are still sold in branches. It also tends to be easier to entice customers to use several such products in person.In moving to an online-only model, banks risk losing their most profitable clients while gaining the ones who are most promiscuous in their relationships with banks, reckons Andy Maguire of BCG, a consulting firm. Until online banks become better than the traditional sort at selling things like mortgages and getting their customers to make use of multiple offerings, they are likely to remain exciting experiments that appeal to the young and technologically adept. If they get it right, however, the threat to the bricks-and-mortar banks will be serious.