It wouldn't be polite to call them an idiot though would it, especially to their face. Isn't that the 'respect' bit, a version of politeness? Wouldn't you subscribe to that much? You can give them your reasons why they are wrong (or you think they're wrong). They can change their mind if they want. And anyone looking on can draw their own conclusions. You might privately think they're an idiot and roll your eyes and give up trying to change their opinion. It doesn't do much good to call them an idiot and start a punch up?
But it's not like the tma forum hoho, in real life should you not generally try to maintain politeness? Respect doesn't imply you agree with their crazy ideas. Just that you accept their right to hold them.
Your point about the number of people holding the idea though, that's relevant somehow isn't it. I can't quite work out how yet.