This is still the most common mistake among newbies. People buy their bitcoins on coinbase, circle, or any of those sites, and they almost never do any other research than that, they think they are set and it's safe to leave the coins on there. Those people will learn the hard way.
Yeah, but sometimes it's hard to expect that someone as big and reputable as MtGox was will quit on you.
But of course no one should keep all their coins on an exchange. In fact I think not many people do that, but they still lose large amounts even if that was not all they had.