Draining only the radiator only will drain out different amounts of coolant depending upon the engine and the location of the thermostat. The normal coolant change interval is 3 years. Now if you drained the radiator once a year and refilled it with a 50-50 mixture of a top brand antifreeze that is approved for use with aluminum, then you probably would be OK. Remember that MB says all of it every three years.... Your choice, a little maintanence is better than none at all.