Do not need it where I live in Indiana. Your tow vehicle covers it and any damage that may come if the load is lost to others property. It does not cover the load itself. So if I were driving and my tractor came off and went into a church full of nun and children my insurance would cover any damage done to the church, nuns, and children. My tractor would be a loss. This all assumes the load was secured properly, truck was rated for the task, ect, ect.
Also if I were to take my trailer to my sisters house and drop it off and it was stolen, I would be out. Now if it stolen on my property its covered by my home owners policy. I looked at getting a policy on my tractor and trailer but in 8 years I would have paid over $15,000 in premiums. My tractor and trailer with all implement only cost $18,000. I will take my chances on some odd ball thing happening.
Chris