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.