Agreed, I was speaking with Greg from Dixons muzzleloader a couple weeks back about this same thing. I got a trade gun off him and asked if it would be periodically correct if I was to put a rear sight on it. He said even though they didn't mostly come with a rear sight a lot of guys would cut a V type valley to line up the front sight with or even put a rear sight on. Gregs dad showed me a gun where he took a chisel and made a hump for a rear sight. Once he had an idea of where it was grouping he then filled in a V in the raised metal.
About ten percent of known trade guns are fitted with some sort of rear sight. Many were a flake as above, some with braised on or dovetailed sights. Rear sights on smoothies goes back to match lock days.
Today's 'no rear sight' rule in trade gun matches is an attempt to give a level playing field to people the have average trade guns, not to reflect real historical gun match.
I put a rear sight on my smoothies, it does not make the gun more accurate. Guys with sans rear sight trade guns can rival my rifle shooting at fifty yards. I do find it a lot easier to shoot, so it's nothing more then a handy crutch for me.
The reverse is also true. My dad had an Ithica Deerslayer 12 ga with rifle sights. He smoked many a covey of quail and scores of ducks fell to it under his hands. The guy he got it from couldn't hit a gallon bucket tossed in the air.