From my owner's manual:
"When To Use TOW/HAUL Mode
When driving in hilly areas, towing a trailer, carrying a heavy load, etc., and frequent transmission shifting occurs, push the TOW/HAUL switch to activate TOW/HAUL mode. This will improve performance and reduce the potential for transmission overheating or failure due to excessive shifting. When operating in TOW/HAUL mode, transmission upshifts are delayed, and the transmission will automatically downshift (for engine braking) when the throttle is closed and/or during steady braking maneuvers. "
So, when pulling my 2600 pound snowmobile trailer, I don't have the "frequent transmission shifting" so it isn't needed. When I'm pulling my 7600 pound travel trailer I do use it. As others have said, there are many more reasons for the tow/haul mode than gas mileage.
Pulling the travel trailer last summer I also noticed some things while experimenting with on vs off. I think that the ideal that you get worse gas mileage with it on is a false perception. I'm talking about with the travel trailer, not the light weight snowmobile trailer. So with to/haul on, the engine seems more reluctant to shift into higher gears and is often running at a bit higher RPM than it would with tow/haul off. Many people seem to operate under the assumption that higher RPM = more gas used. But my experience is that this isn't always the case. I've often seen the instantaneous fuel economy number not change when the truck down shifts and runs at a higher RPM. It seems to me the T/H mode is keeping the engine at an RPM where its more efficient. In my experience, I've seen no perceivable difference in economy between on and off when hauling the heavier trailer.
