The problem is that the die manufacturers don't know how thick the case necks will be (up to a point) , so they made the sizing die suitable for sizing the THINNEST possible neck thickess down to a size that will adequately hold the projectile. The expander ball's function is to size up the thicker case neck ID's to a point where the bullet won't be distorted upon seating.

If you want to minimize cold work on your cases get the Redding type "S" dies that have changable case neck sizing inserts. I still run a ball in conjuction w/ these just to round things up in case of dents and such, but the cases are worked less, stretch less, last longer, and produce most excellent accuracy (in those rifles that will achieve sub 2/3 MOA or so the difference is particularly noticable).

It just depends on what you're trying to achieve whether its worth the small expense and extra effort involved.