Cooling systems are designed so that the engine will run at a designated temperature. The size of the radiator etc. is adjusted so that the engine will run at these temperatures. Again the outside air temperature is taken into account. In most cases, no thermostat is needed.
Most motorcycle thermostats are remotely mounted. That is, there is a length of hose from the cylinder head to the stat. Not a big problem in summer operating temps, but could be in the winter.
Depending on the length of hose, a lot of coolant temperature can be lost thru the side of the connecting hoses. You could end up with the coolant in the engine being too hot and the stat not opening because of this heat loss.