You wouldn't if you didn't want the most energy available pushing your bullet out of the tube. In a neck sized only case more of the energy is used to push the bullet out of the barrel resulting in faster speeds, with a full length resized case, energy is lost expanding the case out against the chamber walls taking away some of the energy that could be used to push the bullet down the barrel, full length resizing also in my opinion causes the case to fatigue earlier due to actually bending it in and out of a different shape. A fire formed case is supported by the chamber walls and that results in more energy behind the projectile. 1000 yard bench rest shooters have done it this way for years, and that is where I was taught this method back in the 70s & 80s they were the record setters with this method. I have very good accurate rounds with this method and as the saying goes, if it ain't broke don't fix it. Everybody has their favorite ways of doing things and you will always have someone saying that " this new way is a better way" and the new way usually is money in some ones pocket, and it won't be yours. I do full length resize my deer hunting rifles because the rounds are used in different family members rifles. Neck resizing only here may very well result in rounds not chambering properly. The magnums loads are one gun specific loads worked up to near maximum loads for that one gun only, those are the only conditions that I will neck size only for, its all about accuracy at longer ranges.
