I'm sorry, but that doesn't actually make any sense at all. Why would forcing a bank to focus on either investment banking or commercial banking, but not both, avoid runs on the bank? Bear Stearns was an investment bank; it faced a run on the bank. Indymac was a commercial bank; it faced a run on the bank. So far, it looks like the banks that were exclusively one or the other have faced runs, but I don't know of any bank active in direct lending and capital markets (and merger advisory and such) that faced a run on the bank.

Frauds and undisclosed conflict of interest still occur, but that is a matter for the SEC, which oversees investments.

This tends to reinforce my "It was already illegal, so making it illegal again is at best a duplication of effort, and at worst a way to further restrict those of us who aren't criminals," point.