In the past 10 years we've been towing, only once did we leave the tow vehicle home, and during that trip we kicked ourselves for leaving it home. I thought maybe our New England trip would be the exception, but we towed it anyway. Sure enough, we were glad we had it.
We have concluded, there is almost never a reason to leave the tow vehicle home. The benefits of bringing it has always outweighed the benefit of leaving it home. I think the only exception would be a trip to Alaska because that trip would be in a constant forward-moving direction. It would also be easier to utilize ferries.