Mt Gox is not a bank, it's an exchange.
Mt Gox is an exchange, but it also provides service beyond just allowing you to exchange currencies. It also serves as a wallet for many people, which is akin to a bank.
Why would I want to deposit bitcoins in a bank in the first place?
There could be a lot of reasons. One would be if you don't want to run the Bitcoin client to send Bitcoins, don't want to go through the hassle of having a paper wallet, and are concerned about security. Another would be if the bank could provide you with additional services, such as being able to exchange your bitcoins at any time without waiting an hour for confirmations.
But the reason I bring it up is to address an argument whose premises I don't agree with. If you think bitcoins are inevitably going to deflate and if you think that's going to present a problem, then the solution will be fractional reserve banking. In the event of some incredible global catastrophe, the bank can pay you back with other assets greater in value than the bitcoins they owe you. Boo hoo. If deflation is a crisis, then this miniscule "risk" is negligible if we can avert a crisis just by taking it.
If people would avoid a currency just because it had this kind of risk, nobody would use any national currencies. They have inflation risk.
A crisis that an economy might suffer from time to time. I didn't know that as a depositor I could be obliged to be reimbursed in anything of value other than what the original deposit was.
This is silly. A crisis is, by definition, a rare event. In a crisis, you might get paid in gold rather than bitcoins, but you wouldn't lose a penny. Oh, that's awful! Forget this entire economic plan because it can't give me bitcoins in the event of nuclear war. Are you serious?