I run a transom saver on my boat, and have on every boat I've had. I do believe they take pressure off the transom, and also believe that with the motor tilted up, the stresses on the transom driving down a bumpy road "could" be harder on the transom than hitting waves running across the water. Think about it, what part of any boat has the smoothest ride? Right, the seat closest to the transom. What part of a school bus has the most bouncy ride? Right, the rear seat that hangs way out behind the rear axle. With the motor down, on the water, the stress on a transom would be mostly vertical, while the stress trailer with the motor tilted up would be magnified due to the leverage of the lower unit sticking out, and it would 'try' to twist on the transom.
For all of the above reasons, I use a transom saver. I too don't think the motor mate type products will do much to protect anything but the trim cylinders/seals. The weight of the motor is still 100% on the transom, only it's tilted up, meaning more leverage to twist.
Now, is any of it truly necessary? I cannot say. Does one truly work better than another at mitigating damage to a transom? I cannot say. But I know it doesn't hurt anything to have my transom saver in place while trailering, and it takes a whopping 10 seconds or so to put it on when leaving the lake. On top of all that, my boat came with one, I bet if I ever had an issue with the transom, the first question I will be asked by the manufacturer is if I used a transom saver. Something tells me saying no would result in a voided warranty.
If you're not living on the edge, you're taking up too much space!