I have ALWAYS had 4x4 because my truck plows snow when its's not towing my trailer in the winter..
I have only needed 4x4 2-3 times towing, BUT having it was priceless !! And it saved me a TON of money and or aggravation those 2 - 3 times...
Sometimes that wet grass on a small up hill slope can be a bugger to push your trailer up in reverse! Or heavy rains on grass and heavy trailers do not go well together
** I think Cummins12V98 has a good picture of wheels and tires that were down DEEP in mud.. I believe that 4x4 may have helped there just a bit !