I find tow mode useful when the engine is cold and you are taking off, towing or not.  Yesterday, I got our Phoenix Cruiser out of the storage garage about three miles from our home.  When the engine/transmission are cold it seems like it really struggles to make a hill outside the storage place.  This clears up after a couple of miles of driving when normal operating temps are approached.  The tow mode seems to help until then.  Otherwise, I only use it like all the others have commented:  downhill for braking.  It is marvelous on some Colorado passes coming down the far side. 
Paul