HI pt, maybe I didnot catch your question but the reason for using assumed temperature is to decrease thrust in order to save engine life and manteinace cost
ATM (assumded temperature) is always higher than OAT and therefor if we are cheating the engine is because we want to use less power. The higher the ATM the lower the to thrust.
I do apologize if you already know this, it is possible that I misunderstood you. Regards.