Easy. 520 chainsets typically are recommended for 750cc and down motorcycles by the major chain manufacturers. If you put a 520 chain on a liter bike, there's a chance it will never give you trouble. There's also the chance that it will give, sending the chain hurtling through your crankcase at speed. Believe me when I say that holes that big are not repairable with JB Weld.
Sure, racer guys use 520s. However, they also change out their chain and sprockets every race or two.
It's up to you of course, but I've always been of the opinion the weight savings in the chain/sprocket area wasn't enough to make it a chance I was willing to take.
__________________
|