The reason is simple. Many off the shelf rifles have loose chambers, and many dies have tight chambers, so when you adjust your die for the shoulder to touch, you are reducing the diameter of the case at the neck needlessly, and causing more brass flow than firing the round. You also then get a sloppy fit that is less accurate. This will often result in brass failure near the base, which is dangerous.How long your brass lasts often depends on how hot you load it. I have several boxes of 300 Win Mag, that was loaded a bit less than full house, with 9 loadings.When I start to see neck cracks, I replace the box. I have almost never had brass fail near the base, because I don't over work it.