in fact some bank do research about blockchain technology, in future i think bank will use blockchain system for his own transaction, but of course bank will be hold the private key.
Well they have the reason for it to hold the private keys since they are institution. It's not different with the current system just in another form because of blockchain technology.
It's a good service if bitcoin will use blockchain system. No doubt banks are helpful in many ways. There are just banks who giving dirt to this reputable institution and don't follow banking terms.
the blockchain system will help bank for secure investment, the transparancy in bank can be accesed by everyone, so people will know where his money used by bank. also this system will help the transparancy loan system.
bank need interest to pay his service, he need money to paid salary his employer, so with this system bank can take interest to each people who borrow money at bank, and use the interest for each cost of activity bank.
In our country there is no transparency on the part of the bank, because we have this bank secrecy law which I think it has been implemented all over the world, banks should protect our personal information and our transactions on deposits/withdrawals and loans to protect its clients.