I use it ("decks awash"..."hull down", whatever) if for no other reason than "realism" in the way that I operate my boat. I certainly agree that it's not easy, maybe impossible, to determine if it actually does make a difference.
One could surmise that it does reduce the surface area of your boat that is "visible" to the enemy. To what extent that works is probably debatable, and if it does work it is likely very much affected by the wave action. I have to believe, or at east suspect, that the amount of "visible" surface area of your boat is only that which is above the surface of the water. How fast the game engine can calculate that is a question, in particular when the "surface" of the water is itself a dynamic, ever-changing variable. Could be an interesting discussion topic...and a never ending one...I suppose.
But then, how many angels can dance on the head of a pin?
