You can trade BTC/LTC and BTC/ETH as well, though, and all of the cryptocurrencies can move into/out of them at will. There are steps they could take to ensure

You can trade BTC/LTC and BTC/ETH as well, though, and all of the cryptocurrencies can move into/out of them at will. There are steps they could take to ensure liquidity in USD, but they're relatively expensive. I, personally, prefer to keep my dollars in dollars and in an FDIC insured account.