In his book Limits to Capital, David Harvey points out what the positive side of banks are: to use the surplus a community has saved; to fund projects for groups that have good ideas but do not have the capital to start their project. In the era of finance capital, the only banks left that do this at all are community banks. But as this article explains, community banks are in trouble and are in danger of being swallowed up by larger banks. Public banks could provide cover for community banks.
Image from Occupy SF