You are conveniently forgetting the conversion to and from USD also involves transaction fees. Not only that, but you also have to trust the trading institution that is holding your dollar balance to not abscond with your dollars, otherwise you have to ALSO transfer your dollars into and out of the institution immediately.
So we are now talking about three transaction fees at a minimum, and five transactions if you don't trust the institution doing the dollar conversion. Plus someone has to eat whatever change in trading value occurs during the period where the transaction is being stored in BTC.
This immediately causes numerous problems, not the least of which being that the trade value of BTC is no longer deflationary if nobody doing real commerce is holding a balance in BTC. In addition to the fact that it won't be deflationary anyway because there can be any number of crypto-currencies in existence.
We are considering the question of volatility and whether it matters. The answer is: Yes, it does matter, and for some obvious reasons.
Consider the two holders of BTC. The speculators, and those trying to use it for commerce. You need stability for commerce-users to hold a BTC balance of any significance. Without stability the only people holding BTC are the speculators. When speculators are the only game in town, instability is guaranteed.
Now consider the so-called deflationary property of BTC (which is a phantom property in my view... wishful thinking at best). What happens to the two holders of BTC if you actually get deflation? What you get is hoarding by the commerce users (i.e. it stops being used for commerce) and more speculation by the speculators. Result == even worse volatility. Hoarding can easily destroy any currency as has been proven over and over again throughout history.