One of my sisters believes all that nonsense about 9/11. But she believes it because she is hard core Democrat that hates Republicans.
For me, I don't believe any conspiracy unless proven true, and never believe science unless it is testable. Though if you can't test it, it ain't science to begin with, it is just a guess.
I could see people thinking Earth is flat, nothing I would bother argue about unless they were like "The Earth is flat, we should spend money on ____ to make it round!"