It's clearly a weakpoint and that would be why governments wants to regulate them out of existance.
Until we find a way to prevent people from storing huge amounts of Bitcoin in one place, we will keep on having the same problem. If a hacker needs to choose between hacking a person with 0.2 Btc and a wallet service provider or a exchange with 100 000 Btc.... what will he go for?
It's also a good excuse for them to run with money. {In-house job}