As pointed out above, the shellholders are for headspace adjustment, not neck tension adjustment. If you don't know the difference, you need to stop reloading, read more, and understand the difference. Neck tension is controlling the diameter of the neck prior to seating (how firmly the neck is holding the bullet... usually stated as the difference between the bullet diameter and the internal diameter before seating the bullet. It typically runs from .000" to .005"). Headspace is determining the length of the case from the shoulder to the case head (or alternatively, the left over space between the case head and the bolt face when the round is chambered... that measurement typically runs from -.001" to .003").
I don't use the shellholders as I have a Forster Co-ax and a Dillon 650. If you need to adjust headspace, it is just as easy to screw your FL die in a little more. The Redding Shellholders will remove some of the trial and error if you know what your spacing is, you can just swap out the holders so that you can get to your target more quickly. My FL die has a micrometer head, so it doesn't really bother me.
Where I think these jobbies might be really useful is if you are loading the same caliber for multiple rifles and each have different headspace. In that case, if you don't have a FL die with a micrometer head (most don't), you can just swap out the shellholder when you are reloading for a different rifle. These things only really save time if you have to set up more than once. I hate setting up FL dies, but I do it once, and I am done until I decide to make a change (I don't have multiple rifles with the same caliber).