You will get more replies from the saw builders, I am certain.
My initial reaction was exactly as you said. "if it was supposed to be there, then...."
I have changed my tune a little. I am now to the
theory.
Some engines were designed for only one ring. Some were designed for two.
One theory was the second ring helped cool the piston. Another theory was the second ring created more friction.
You won't see a increase in compression. I doubt that one ring wears out any faster than a two ring piston.
I would be of the theory that if there is two grooves there, put a second ring on there. In your situation, put one ring on it and run it like you stole it.