What happens if the bank offers to lower your rate if you give access?
What happens if you cannot find a bank that gives you a loan without insisting that you give them access to your accounts?
This is what I find so frightening about the Facebook-login-only web services.
It's the first step towards the widespread requirement of a massively asynchronous information+value exchange between customers and businesses.
Then you either go to another bank, or maybe you don't get the loan from any bank. If you don't get the loan, you change your plans to save up cash, or you change your goals.
Either most people will go along with this (likely), or banks will have to decide if a smaller loan pie is good business.