Banks are massive, massive institutions, and it is hard enough to coordinate people and resources within them, much less across them.
Back-office operations are indeed cumbersome and a major cost-center, and there is much value to be captured in simplifying all of it. If "blockchain" can serve a Schelling point to help get right people in these institutions to talk with one another so as to get on the same page, then there's a chance that some progress can be made towards solving these problems.
It's "blockchain" because "blockchain" is the new hotness, that's all. Any real world, inter-dealer distributed ledger will, in all practicality, be more like a cluster of SQL databases backing some kind of state-machine replication protocol than a cryptocurrency.
Happy BTC10k, by the way.