Well, I can't say that I'm looking for this behavior all the time. In this case I was using the brick generator and figured if I was going to give the bricks some various greyscale values for the height channel I might as well pump that into the roughness. This seems easier than adding a second brick generator and then having to make sure a majority of the params align between both instances. Normally (instead) I'd create a base fill layer and then add the brick generator to the mask. The end result seems the same.
Does it make sense for height values to correspond to roughness, no, but it works as a base to provide some cheap randomness

I think this mystery channel mapping rollout has been around for a long time and I've been losing my mind with the inconsistent behavior. As I'm messing with my layers suddenly the options seem different or I make a mental note like, "the brick generator has that channel mapping rollout" only later to discover it doesn't.
This reminds me of the various particle options not always refreshing properly when changing the emitters or receivers in version 1.x. It's probably fixed in version 2.x, but I gave up on using them because you never knew if all the params would be available.