I don't have a rear sight on my smoothbore. I had the same problems with accuracy most people do when I got my first one, went through all that filing and missing for a long time. Pretty frustrating. One of my main interests in studying the ways of the old boys is to learn some of the skills they had, and I thought shooting a smoothbore without a rear sight might very well be one of them, so I never considered adding one. After quite a while, 3-4 months, it all seemed to come together, and I've now used that gun as my main one for the last 17 years. I've killed about 20 deer with it out to 70 yards, shot 4" groups at 100 yards, etc., and I have to say, none of the old skills I've learned gives me as much satisfaction as that one.
It's a personal choice, of course, just depends on what the objective of your shooting is. My way is not for everyone. I have several rifles with rear sights which I could use to collect my deer, but somehow I usually reach for that smoothbore, because there's much more to my hunting than killing deer.
I can't think of a single reason not to put a rear sight on a smoothbore if a man want's to. It was done historically, if that's of any interest, and it certainly does make hitting things easier for most shooters.
Spence