Many car class-A surfaces that get modeled for production are already based on 3d scanned clay models (basically reverse engineering the shapes by hand in the software). The workflow of converting “dirty geometry” to nice Alias surfaces is nothing new. If you create your new surfaces on top of a scanned mesh or on top of a sub-D mesh is no big difference. At the end of the day you will need your clean NURBS surfaces either way for the reasons you mentioned. But there is no need to built those earlier than really necessary.