I fail to see where the confusion lies. Really, what exactly are you people arguing about?
And in general terms, NX means experimental. By the time the Defiant came about, there were numerous new ship classes being pushed into production to combat the Borg and eventually the Dominion (such as the Nova class and the Prometheus). But back when Enterprise got kicked into space, Starfleet had invested all of its eggs into that one basket. A basket which heralded a new era in space-travel. So it'd've made sense to call in the NX-01.
It was also the first ship Starfleet produced after it decided to get a little more firm with the Vulcans.