Bayonets were used on military shotguns in WWI & WWII. They looked mighty fierce, with that L O N G bayonet & 20" bbl. Probably not the most practical for HD if you have narrow corridors.
OTOH, I really enjoyed bayonet drills while in the service. Stab, slash, butt stroke...it's all good. Beats the heck out of harsh language if you are out of ammo or the goblin gets up close & personal.
The Mossberg 590 with bayonet still seems a bit long for HD. Ideally, I think a 14" bbl and the M7 bayonet would be the way to go. Of course, you get to pay $200 for the privilege of owning a SBS, not something I am willing to do.
I have no doubt the bayonet would play havoc with a slug's zero, as does the bbl/mag extension clamp. I wonder if the effect is even noticeable at HD ranges? Another question is the effect of the shot pattern at HD ranges.
So, I won't laugh at the notion. Wiser men than I have thought it perfectly reasonable for their application.