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.

You can keep your wallet file secure. Encrypt it, store it in a pen drive and secure it as you want.

The decentralization has more to do with the fact that no government can control the currency.