I would be quite worries about that happening if I was running mt.gox. Basically you have an office with $15M in cash lying around (considering mt.gox controls 500k BTC). Anyone can walk in and force mt.gox to transfer all those BTC's to his wallet and there is absolutely nothing he can do to either get those funds back or try to find out the identity of that thief.
A normal bank doesnt have $15M in cash in their vaults and they are much better secured then 2 nerds in a communal office building.
In all fairness to MtGox, most banks have little more than old/retired people serving as 'armed security', and often don't even have that.
Granted, they do have vaults. But how is that different from storing wallet files on encrypted/removable drives? One takes a combination to open. The other takes a password. A criminal could conceivably 'force' a banker to open a safe, just as much as force MtGox to decrypt the wallet file.
