Bitcoins are only as valuable as the currency they're tied to (USD). That only invalidates the necessity of using Bitcoins. Why not just use regular money, accepted by many countries around the world and backed by the largest economy on Earth?
This is incorrect. Bitcoins are only as valuable as what people are willing to give you in exchange for them. That could be currencies, but it could also be services - Look at the various funding options available on any of the exchanges, and you'll find that while most of the transactions happen in USD you can in fact use a variety of currencies to get in, and that's not even to speak of the rapidly expanding variety of goods and services you can find on the variety of auction/classified sites. I sold someone $10 face value in 90% pre-1964 silver coins for 1.5btc/1$face, is that a USD transaction?