Some reasons not to use just one bank :
- A bank employee commits some act to move your funds to another account (either by mistake or from criminal intent)
- You are scammed by some electronic means (via a compromised debit card most commonly, normally would only affect one bank)
- Your debit card is lost, expires or malfunctions while you are away from home area. (you can use your card from the second bank)
- When I was younger I had accounts at many banks. This was to build up a reputation at each, so if I needed a loan I had options for where to obtain it.
- Allows access to more credit cards. This is especially important as I have had cards expire while overseas and was saved by having several more to choose from.
All that said, I keep the immigration funds in one account (just eliminates a "point of failure" when dealing with immigration), and do not use that account for anything else. We have all seen posts of mistakes made and people dipping below the required funds.