Also, remember that banks work with cash. There is no built in mechanism to force a borrower to repay a cash loan, but there is a legal structure, which works just as well with Bitcoin.
What do you think about the weaknesses of fractional reserve lending? (https://www.youtube.com/watch?v=1HYSMxu-Dns)
I think these comments are interesting: http://www.quora.com/How-does-bitcoin-and-the-information-technology-that-makes-it-possible-have-implications-for-our-world-as-a-global-society