The bigger problem here is not whether it is ok with them or etiquette, but rather the fact that you are taking a huge risk on someone unknown for no particular gain whatsoever. I have done it a times to take a little breather for a few min and then pass. I also have almost crashed doing the same when the idiotic person I was just drafting decided to slam on the brakes because he saw someone ahead. There are all kinds of idiots driving and doing crazy stuff behind the wheel. Same goes for bikes!!
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Arguing with ignorant people is an exercise in futility. They will bring you down to their level and once there they will beat you with their overwhelming experience.