As the activities and risk profiles of large banks and securities firms have become increasingly similar, and as financial intermediation has run more through securities markets, we've certainly learned in the past month or so that it is not only commercial banks that can threaten financial stability.
So we must worry about excessive leverage and susceptibility to runs not only at banks but also at securities firms. To be sure, investment banks are still different in many ways from commercial banks. Among other things, their assets are mostly marketable and their borrowing mostly secured. Ordinarily, this should protect them from liquidity concerns.