My experience is that tire dressings do help. Maybe it works like sunscreen. I put 6 new tires, Mastercraft HT on my dually (back in '06) and used tire dressing (aerosol spray on kind). Over the course of the next 7 years with tire rotation (only put on 5K per year) one side of 2 tires never got any tire dressing. In the end their last position was inside rear.
I guess that side never wound up being OUT at any time. But 4 of the tires did get dressing on both sides. The untreated sidewalls cracked badly and one split and blew out while I was backing into my driveway. The rest that had been treated had small cracks but nothing bad.
Sad part was they died of old age and not from wear. My new Transforce HTs got tire dressing on both sides when I installed them.
Besides the "dressing" lesson I also decided that rotating tires on a low mileage dually is a waste of time. Its cheaper to replace the front tires when they actually wear out.