Its an old custom, something I was told to do when traveling two lane roads was the rule rather than the exception. I am sure many of the younger RVers have never been told about the flashing light thing, some of them might even think you are warning them about a problem, or may take it as a derogatory signal.
Also, my current RV, the Tiffin Allegro has an acknowledgement switch at my left hand position next to the transmission controls. Really easy to get to. With the previous RV, the Adventurer, the light switch was on the far lower left of the dash. It would require that I look down and reach down to toggle the switch. Thus, I seldom used the acknowledgement.
Happy Trails.