$11m in the bank is just numbers on a hard disk too...
Don't get me wrong, you do have a point - the downside of acquiring large investments in bitcoin is that you have to take security and backup very seriously. The bigger downside right now is there's less in the way of legal protections - although there are online accounts to hold bitcoins that might seem analogous to banks, there's no legal protection, plus 3rd parties are riskier if they get hacked (since money can be stolen virtually, and again, unlike a bank, it will be your money being stolen, rather than the 3rd party's). Similarly I suspect that no contents insurance policy will cover any loss in bitcoins.
But it doesn't seem an unsolvable problem. Surely any geek already has a rigorous backup plan (and with that much money, you could sell a small amount to raise any required hardware cash), and is clued up on the use of encryption tools like Truecrypt?
And it seems a smart policy to spread investments around. If they have $11m in bitcoin, and little elsewhere, that does seem stupid. If they're just stupidly rich people, with bitcoin being one of many investments, that doesn't seem strange (even putting all your money into safe bank accounts has a risk, if it's all in one bank).