In defense of Willarddcker, I believe his purple nacelles were inspired by production photos from Star Trek:TMP. I have seen some of them myself and they did indeed have an Enterprise with purple nacelles.
But if this isn't the reason, and he just used this color on his own, I still like it. It is different and I always think variety is a good thing.
In fact, willarddcker has created quite a few Constitution/Enterprise Class variants. It's my hope that he releases them all at some point. He has not realeased (I think) at least 3 versions of the Phase II, a.k.a. Fontana, 3 Terran/Earth Empires, 2 Vanessa E. Steggs, 3 Khambattas and more I know I'm forgetting.
And every darn one of 'em is a work of art!