Several different things about just neck sizing. Main thing to remember, brass has to be used in the same gun it was fired in. If you shoot 2 different guns, in the same caliber, you'll somehow have to mark the brass, to keep them seperate.
Now, onto why. Your working the brass less, by just neck sizing. So it'll last longer. When I neck size, I only size about half the lenght of the neck. In theory, the case and bullet are centered in the bore, so better accuracy. Now, depending on chamber demensions, and how hot or mild, you load, you will have to full lenght size eventually. You might get 3-4 loadings before you have to, or, you might get 8-10. It will vary with different guns/brass,etc. Hope this helps alittle. gypsyman