There's some truth and some misconceptions here.
Temperature measurement is useful if done correctly.
You always need to measure the same location on the engine in order to know if the engine is running hotter or colder than a previous run. There is a wide range of temperature distribution on any engine. You can't easily compare engine temperatures to some standard "good reading" without knowing that both were measured in the exact same location and under the same conditions (wide open throttle or was it idling, how much time after how much throttle etc.). I don't try to set needles by engine temperature, although I understand that is the procedure on full scale planes with exhaust temperature measurements. EGT changes rapidly with carb changes. Engine temps change more slowly because of the thermal mass. This lag in temp response would make it difficult to set needles based on temps measured at the cooling fins.