So are puka shells. And bitcoin's exchange rates are fluctuating pretty rapidly, especially since exchanges tend to randomly disappear. What happens when you have $5,000 in bitcoins, and the bithurf.com goes down and suddenly you only have $500 in bitcoins?

>And bitcoin's exchange rates are fluctuating pretty rapidly

Not that rapidly. When pricing in Bitcoin most buyers are okay with prices being based on recent pricing. Add a bit of padding to your Bitcoin price to allow for normal fluctuations and you're fine.

>What happens when you have $5,000 in bitcoins, and the bithurf.com goes down and suddenly you only have $500 in bitcoins?

You wait a couple of days for the value to go back up (at least that's how it played out when Mt. Gox crashed). Significant non-short-term pricing movements in Bitcoin land don't happen overnight.