I don't see the point unless you can get the ring made for free. Like Phil, I pretty much fill the ring and then shut down the vents to save charcoal. I can see, however, that if you do try to use just the right amount of charcoal for the cook, that a smaller ring would be good. The charcoal doesn't burn well if it's too spread out and the layer is too thin.
Let us know how it works and how you like it.