I keep getting told this, and I did have problems with neck sizing when I first got my Blaser but they turned out to be down to a faulty die. Since then I've been neck sizing with a Lee Collet die and have had no problems at all. I don't know how many loads some of my brass has had since last full length sized but it is certainly more than 5 in some cases and I've never had one fail to chamber. My loads are fairly stout.
So, I am sure there is a good reason why people keep saying this but to date I haven't found any practical reason for it in the course of my own reloading.