I'm curious how you pitched yourself to YC. Did you say you were going to be a sort of venue for people looking to build custom financial services on top of your API?
To do so, we built an API first. Rather than waiting to release our mobile and web clients to serve traditional banking customers, we decided to release our API first, so that any companies that can take advantage of it can get after it. We've talked to a lot of companies that want to automate ACH payments, wires, check sending, and so on, rather than relying on manual and offline processes. Most businesses will be best served by our apps, once they're available.
As for use cases, our conversations with business owners have led us to believe that nearly everyone is frustrated with the existing banks. Applications are difficult to use, offline forms are required, service is poor, and so on.
Seems like I could build my own personal banking/accounting site securely with this.