Not trimming saves a little time but having crimps that are all over the place is as annoying as having your ex mother-in-law standing behind you telling you how to clean your guns. If necked brass is sized, and trimmed to consistent length before use, then fire formed to chamber they should be very consistent from then on, and if caliber will only be fired from same chamber you can neck size only and have ammo matched to your rifle. Brass should last longer that way too.
Neck sizing is not recommended for auto loaders as I understand it. But it does well in bolt actions for me and lever guns too from what I have been told.
BTW - what is this new brass of which you speak? And where does it come from?