Bobs - way back in the day when dual wheel pickups started to appear on the roads pulling ever more larger RV trailers us truck drivers called them "training wheels" hahahaha...
We have both around summer base. The number 1 advantage to having training wheels is you can run much lower air pressure in the rear tires and enjoy a better ride without having to air up every time you put 1/2 a load in the rear. In addition the dual rear wheels don't fit in every hole in the streets and roads.
They also have better sidewind "stability" when towing "RV" type trailers (and others) with their axles 1/2 way up the trailer and the long weathervane tail out behind.
If you tow a trailer like a gooseneck flatbed or stock trailer with a dual or SRW they track so nice it's more your choice of truck because they don't sway around.
Main disadvantage - rocks jammed between the tires and poor tracking in certain mud and snow conditions.