If you have a good chamber and headspacing is right you can neck size your brass, but if the head spacing and chamber are sloppy or if your press has a problem you will have to full size. A bad press can make the necks bad even if you fuill size.
You make a very valid point. The machining involved will definitely affect the accuracy of each individual hand load. I know that if you get a lot of blow by past the neck it can dirty the chamber quicker increasing pressures. This is one of the reasons I surmise that benchrest shooters make sure there necks hold tighter tolerances and allow for a more uniform release of the bullet. Like somebody else had said, it also lowers the amount of work hardening on the brass due to less movement.
What problems could a press have that requires full length sizing dies be used instead of neck sizing ones?
What ever process you use seems to work good for you. Sounds like you don't get a long life out of brass if you only get accuracy in the first three loadings with new brass and full length sizing. You may want to have your rifle checked or lower your loads for excessive pressure.
Like anything else, all parts of an operation have to perform to the top of its ability. A press that has a worn ram will have a lot of slop. This can do one of two things, and this is all speculation:
1. The brass could be fed into the die improperly not allowing things to line up properly. If not caught in its early stages could wear out the dies in such a way that takes them out of round. Now when you load it into your rifle, you load an oval into a circle. This would feel snug as if it fit properly but then your pressures are off or increased due to the deformation. This would have to happen over hundreds of loadings, but if you don't notice it, the problem never goes away and only increases. Metal when worked enough improperly will move and conform to the force that is being applied.
2. Your alignment when sending the ram home could cause the brass to wobble within the die. This would cause alignment issues when trying to properly neck size, or seat a bullet. This could cause excessive pressure to one side or the other. An improperly seated bullet will not fire uniformly with in the throat.
Again this is all speculation. Guys this is a forum to share information. A new member should never feel as if being under the gun. Your opinion is your opinion, but should never be forced on one person or another. We share experiences and opinions. There are some here that teach us fact. Be careful how you use this forum and the attitude in which you send your information or your opinions. There is no reason that anybody should feel degraded for sharing a thought or an idea.
Good Shooting and Merry Christmas, now lets get back to .264 calibers for kill'in some stuff.