I just finished continuing qual training yesterday. They tell us for the GE engines (CF6-80C2B6F) the reason for the 30 second run-up is to shed ice accumulation off of the stators. They tell us to make this run-up every 30 minutes during the taxi. The run-up before takeoff is also done to 60%N1, but only long enough to see that the engine instruments are normal. Then we may release brakes and go. Seems logical enough.