I think your question is based on the false assumption that there is some 100% safe way to store money that doesn't cost anything. Mt Gox can't stuff your money in their mattress, and even if they did someone could come along and steal the mattress. They have to sensibly balance risk and cost -- zero-risk is not a sensible or achievable goal and is not available for free.
There is always a chance that an exchange will not have sufficient liquidity to meet withdrawal demand. When that happens, they have to process withdrawals more slowly. But the alternatives are more expensive and would mean the exchange would have to charge higher commissions. Exchanges like Mt Gox have strong incentives to get this balance right -- a reputation for slow withdrawals will hurt their bottom line and, of course, high commissions drives away customers.
So you can tell how good a job they're doing by looking at how their customers rate their withdrawal experiences and what their commission rates are. If you think they're not balancing cost and risk right, use a different exchange.