I had an opportunity to run a set for a week, while my truck was in the shop. The boss had just taken delivery on a new truck that had them.
They do reduce the curb weight of the truck by a slight margin. They also give a slightly more comfortable ride.
However, that comfort goes out the window when you consider the possibility of a blow out.
When running duals on a tandem setup and a tire blows, the axle stays level and you have enough control to continue, preferably at a reduced speed, to a safe place to pull off the road.
With super singles in a tandem setup, if a tire blows, the rim hits the ground, costing you control and forcing you to stop now. Regardless of whether it's safe.
Now, remove one of your drive axles. A blow out on a single drive tractor, pulling a trailer of any consequence, will dramatically increase the chance of loss of control which could lead to a jack-knife or worse.