The only role banks fulfill that couldn't easily be replaced by a digital currency is financing speculative investments by providing loans. Unfortunately loans also expand the money supply (and contract the money supply upon repayment/default), resulting in a positive feedback on investment, which can induce instability (i.e. the business cycle).

It would be much better to decouple the money supply from investment. The money supply could be controlled either by a trusted agent (e.g. central bank), or possibly by a built-in stabilization algorithm (an interesting avenue for research), while investment could by financed privately. This would be the best of both worlds.