James,
I think you aren't taking into account that whoever has the NXT doesn't want USD's. If I had a large vestment in NXT (which unfortunately I don't), why would I want to trade it for an inferior currency like USD's? To get me to come of off my NXT you are going to pay MY price. If any other currency other than the NXT currency is given influence like this in the NXT ecosystem, it would automatically denominate the NXT currency to a point of weakness in terms of value for negotiation and purchasing power. NXT currency should be the dominant currency in the NXT ecosystem (imo).
USD is currently the world currency. Everyone know it. That's why people want to use it, even if it's "inferior".
NXT will be the dominant currency in the NXT ecosystem no matter what, because that's just logic. But we need to let people make their choice and don't enforce our currency.
I'd like to repeat an analogy I mentioned the other day in here which I liked, which is to compare the NXt economy to a Prison. Nxt AE is the Prison itself and within this prison the prisoners have all kinds of needs and they trade amongst themselves. A dirty prisoner will trade his cookies to a hungry prisoner for Soap. That prisoner might take half of those cookies and trade them for toothpaste. These trades require trust because during any of these trades one of the prisoners might pull out a shank and just take your stuff.
NXT the currency however is like buying Cigarettes from the Prison Commissary (In our Prison the Commissary only sells Cigarettes) . When you buy NXT you are getting it from the institution, so you don't have to worry about trust or getting shivved. NXT being a math based digital currency requires no trust at all. Commissary cigarette prices can still fluctuate on their own based on supply and demand and so it will be with NXT (the digital currency).
Once AT is implemented you'll also be able to buy Soap, Toothpaste, etc with a guard present so at some point NXT/Cigarettes will have to compete with everything else to remain as the reserve currency, but since NXT (Cigarettes) are so awesome (addictive) and has properties that make it very easy to trade (lightweight, comes in packs, small size) it has little to fear from other commodities that are not so easily traded.
it's not a perfect analogy I know, but I think it shows how Nxt should be like a little self contained economy and NXT the currency should have to compete for dominance within it but will likely become the reserve currency because it will be universally accepted.
What would guarantee its universal acceptance? If trading anything on the network for USD's is allowed the only guarantee is that NXT will be the currency that fees are charged in. I think this would only serve to make the NXT currency less accepted and less in demand. I agree that NXT should have to compete for dominance, but as a platform on a whole; and the base currency of that platform should not be undermined by allowing other currencies that level of influence within the platform.
If you look at the reason that the USD is so strong, it is because of legislation that made it THE legal currency for a strong economy. The USD does compete with other currencies for statuses of worth and purchasing power and rate of exchange, but it is allowed a high degree of no competition in its native economy. if you want to buy something with Euros, you have to buy from Europe. If you want to buy something with Loonies, you have to buy Canadian. It should be the same with NXT. If you want to buy something in the NXT ecosystem, you should have to do it with the native currency.