Mt. Gox was mostly a BTC payment processor and exchange between BTC and "fiat" currency (I put "fiat" because BTC itself is fiat as well). While you could leave money there (both BTC and USD) it was mostly to buy/sell BTCs and not to keep a balance like you do in a bank.
People who put their actual savings in BTC should do so in a wallet, which at least you do have and keep yourself in your PC. (Hopefully you back it up every time you do a transaction!)
Hell, I learned the lesson on not keeping money on intermediate entities years ago, during the Second Life Banking Crash of 2007 and its fallout. Used to have my Linden Dollars deposited at Ginko Financial … until that blew up. We were offered a 1:1 swap into stock for Ginko (Something) Bonds at a then very known stock exchange called World Stock Exchange. Thanks to that, I was able to recover about 33% of the original balance I lost. I said, well, I'll invest by buying stock on the companies listed here.
Then the stock exchange suddenly halted, all withdrawals were "temporarily" suspended. While trading reopened for some months after a 9-month hiatus, withdrawals never were re-enabled… and the whole thing just disappeared sometime around 2009. In fact, the temporary halt page is still there, and it looks a lot like Mt. Gox's announcement, doesn't it? So while I did use Mt. Gox, and I did have a meager balance there (something like 0.000007 BTC and 0.0007 USD) I never kept large amounts of money there for more time than necessary. And it seems I was right!