If I had any bitcoins hosted on mtgox and, for some reason, had not already taken them out, I would do so right now . When you give them your bitcoins, you are trusting them to keep your money safe. I trust my money with my large bank for two reasons: (1) they have a large safe and have practice keeping people out, but more importantly, (2) if someone were to break in and take some of the bank's money, I would know t…
I would know that I could still withdraw my money because they have enough cash on hand for me to do so.
http://en.wikipedia.org/wiki/Fractional-reserve_banking
Or if you want a more practical example, keep watching Greece (or look at what happened to Argentina 10 years ago).