More airflow can't hurt. Especially in hotter weather
More airflow THROUGH any size radiator or any radiator in any condition will result in cooler coolant temperatures. regardless of riding speed. Low speed riding does not get very much air THROUGH the radiator.
So is it worth buying one if mine doesn't have it on already??
It probably will not help much if you spend very much time idling around in 1st gear. It will help it some but how much it will help depends upon you average speed over 1 minute.
Who idles around in first on a 500?!? 
Most of the guys that are having overheating problems.