Mt.Gox really is an exchange; the whole point of the site is to convert between BTC and USD. If you wanted a bank, you'd use something else. It's also not clear to me that it's safer to store BTC on a server.
It is an exchange, but hackers are having a field day. People are realizing what the world realized many years ago: we need banks. I feel like bitcoin is going through all of the growing pains and making the same exact mistakes we already learned about currency.