You can't really understand how much it does or doesn't hurt to break your arm if you have never had an injury in your life.
Not saying people who have never smoked marijuana can't have a valid opinion, but I do agree that in general, their understanding of its effect will not be as thorough as someone who has experienced it. And like almost everything else in life, there will be some exceptions to that.
As I may or may not have stated, I have friends who smoke. Even though I have never directly experienced marijuana, I can say, without hesitation, that results will vary from person to person. To be honest, I'm not sure exactly what the point of your analogy was - I have never broken my arm, so I don't know exactly how much pain is involved, but I do know the implications of a broken arm, and that it isn't a good thing. Besides, having an opinion and being right are two completely different things. If you are on the right side of an issue, you should have nothing to fear.