Bitcoin's utility as a proxy-USD will remain as long as a USD / BTC exchange exists. Whether it is or isn't a buble doesn't really matter if you're only using it as a proxy. If it isn't a bubble, then hoarders will profit strongly from BTC. If it is a bubble, then hoarders will be punished.
If you don't want to take that risk, just exchange the BTC into USD at the end of every day through MT. Gox.
Forex trading is common to every currency. How else are you supposed to get the currency? If I go to Japan, I can't spend my USD in every shop, I have to find a USD/Yen exchange. That doesn't mean USD is a proxy for Yen.
If you don't want to take that risk, just exchange the BTC into USD at the end of every day through MT. Gox.