I used tow haul all the time when towing my fifth wheel. Now that I have a 14,500 GVW E450 based MH, I will use it all the time too.
My thinking is the tow part is less important than the haul. My unit like most comes in near GVWR before cargo. So the unit is "heavily loaded" from the get go. If the Tow/Haul (as in hauling a heavy load in the truck bed) is for that situation, then it seems best to use it all the time.
I really appreciate the auto shift down when I am trying to stop or slow down. I think it safer for me to not trust my ability to think ahead to remember to turn it on just when I need it.
For completeness, I did not use tow/haul with the pickup usless I was towing. I have no reason to think the tow/haul has much effect if any on the MPG .
Everyone has opinions. These are a few of mine.