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.
But we learned that short-term securities markets can suddenly seize up because of a loss of investor confidence, such as in the unusual circumstances building over the past six months or so. And investment banks had no safety net to discourage runs or to fall back on if runs occurred.