Only in very cold climates will it serve to warm the trans fluid, and that will take some time since the engine has to first come to operating temp to warm the coolant in the rad before it can warm the trans fluid.
many of our newer automatics and CVT transmissions have fluid warmers(and coolers). The main difference between a warmer and a cooler(because to some degree they both do both) is the cooler puts oil to water on the radiator side of the circuit where as the warmer puts the oil to the water on the cylinder head side of the circuit. The cylinder head/heater core side is all pre-thermostat so the water heats much more rapidly. When on the radiator side you have to wait until the engine is at full operating temp before warm water starts circulating.
As far as the fluid thermostat goes, some vehicles(such as pathfinder 2013-current)use this from the factory. The warmer is bolted to the transmission itself, then it uses a cooler in the radiator, then the fluid thermostat controls fluid flow to the auxiliary cooler.