Is there ever a reason to have multiple bank accounts?

Bank accounts are useful for setting up bounces. If you create an account, obviously you have that account on the system, which makes the bank more valuable as a bounce node (dotted line around it; takes longer to process). Your log-in credentials will never be revoked, either.

You could of course hack an existing account, but creating one is much easier and does not start a trace, so this is useful early in the game.

Having a bank account will make stealing money from it much easier, as there is a very brief window of opportunity for you to delete the log of both sending and receiving money. Not having to connect to another bank to do that is very helpful.