I agree, but if you measure your temp at the exit of the radiator and it is too hot then that water is now going to the engine to pick up heat, so when it gets back to the inlet it's now even hotter. By the time your sensor sees cooler temps it will back off on cooling, and start the roller coaster again. If you measure your temp at the inlet to the radiator you start your fan quicker, add cooling to that hot water and minimize the added heat above set point. Feed forward control eliminates most of the trial and error of a feed back control system. Now if you want to really get fancy use both sensors, one on the inlet to the radiator and one at the outlet in a feed forward control with feed back trim. Anticipate the cooling needed and start your fan, then measure how it's doing on the outlet and adjust the speed up or down based on your feed back sensor. Only really effective with a variable speed fan though.... Throw in a little integral action on the feed back loop and you'll run cool as a cucumber.