Spare parts....zero reason to use a neck die, I now FL size everything.
Literally this. I own both RCBS Competition 308 FL die set and the standard neck die set. I recently found the expander (I think) was out of whack. Using the rod and expander, not even the depriming pin, has all this die been good for. I've sorta wanted to perform a little test for myself but I've had such good results so far this hardly seemed worth the effort.
My own little 3d maga level analysis of the reasons for neck sizing just made less sense to me over time. When I initially heard of it, it sounded like a great idea in theory, same exact-combustion chamber volume. Even bought the neck die after the fl die. Then I thought about it some more. My biggest issue I personally had was that proponents of neck sizing only seemed say, or at insinuate, that how consistent one closes the bolt makes a difference and "all you have to do is just close it the same way." Seemed like a poor argument when the alternative, full length sizing, essentially eliminates that human-based variable. I've learned by trial that even the 0.002" bump to is enough that whether you slam it or work it real gentle or mix it up, didn't seem to matter.