It seems to me that the major advantage of neck turning occurs in the situation where you have a chamber that has an intentionally "tight" neck, into which standard cases won't fit.
Then you turn down the outer diameter of the custom case until it just fits (maybe .002 total clearance) and you have a neck that is going to sit rock solid in there, reducing any angular difference from one fired shot to another -- because all the cases are fitting so snugly.
At the same time, you want the cases to be very snug also.
(This is for BOLT action rifles, not semis!)
There may be some advantage to uniforming the neck when you don't have a tight chamber, but it is probably a lot less.....
I neck turn my 6PPC to fit its custom chamber, but I don't do anything with my .223 standard chamber.
hope that helps,