You know, I was thinking the exact same thing a few days ago. It was early in the morning when I was looking at my rifle and wondering how the hell I was supposed to get it zeroed if the rings couldn't be adjusted. Then after a few minutes I remembered what those knob things on the scope do.
Never heard of adjustable rings, but I have seen a few that have cut outs to allow for use of BUIS. Cannot for the life of me remember the brand. I can certainly see a use for extreme long range or precision shooting or special cases, as even a few millimeters adjustment to the rings would result in substantial changes on target downrange and thus allowing a shooter to compensate for huge amounts of bullet drop.