The mixing of commercial and investment banking seems to have had very little to do with the last crisis, no one can quite explain how it would prevent a new one, and it would be a real pain to break up all the banks. The one thing I will say for it, though, is that the Glass-Steagall hard line between commercial banking (deposits, loans, etc.) and investment banking (trading, etc.) is probably a more sensible line than the Volcker Rule line between market making and proprietary trading. There is every reason in the world to combine customer-facilitation and "proprietary" trading in one entity, but there's no particular conceptual reason why that entity should be a deposit-taking bank.