The moving, visible parts, and those things that control it might better be brass - they'll hold a finish better that plastic. The actual drain line - plastic is probably okay. I did some remodeling at my mother's house over Christmas...the brass tub drain came apart in my hand, the pvc stuff was still good. The softer plastic is more liable to get the finish damaged because the plating is hard, and the plastic is much less so. The differences in thermal expansion means the bond won't be as reliable, either. My personal opinion.