The more I get involved with bitcoin and the more I use it the more I ask that same question, why open a bank account? Even more so since the launch of the various bitcoin linked debit cards. Its now just as easy to spend bitcoin as it is to spend fiat.
If you can borrow money from banks for mortgage, then there is no need to open a bank account. The bitcoin related banking system is still in early stages, we have to use fiat bank for the time being.
It's not only that you need bank account for borrowing money, unfortunately you need it for everything else. The Bitcoin ecosystem is still to small. And I have no problem admitting this even as a die hard bitcoiner.
Of course, this will be completely different in the very near future. I expect that in 5 years this changes completely!