I use one after I cut my arrows. Almost nobody can cut an arrow exactly perfectly square, so it gets you to the proper starting point once the cut is made. The better shops will use one after the cut to make sure the stuff that goes out the door is the best quality possible. I then will use the aluminum side to trim the insert to perfectly square, and that helps in getting the broadheads to spin better. If you use the hidden inserts, then there's only one step in the squaring process.
So, to answer the question, yes they are worth it if you are looking to get the equipment performing at its peak. If you are willing to accept a little bit of equipment error, then you're probably fine without it. However, there are so many variables in bowhunting that I choose to eliminate as many equipment problems as I can.
