And a lot of the time, there isn't any harm.
However, sometimes there is:
Note: before anyone gets their knickers in a knot over some of the headings, understand that the site isn't saying that each topic is in and of itself problematic. They're not against all home births or vegetarianism for kids: they're making the point that misinformation, bad decisions, and just plain getting it wrong can be dangerous (I don't know a single vegetarian/vegan of any sense who will deny that you need to have a clue or three about what to eat to maintain a balanced diet...in fact all the sensible vegetarians/vegans I know tend to get royally peeved by stupid people failing at those diets and then blaming the diets instead of their own ignorance...nor do I know of any midwife worth anything who would ever condone a mother bleeding to death over calling an ambulance...they are citing BAD examples of people using beliefs to trump sanity and are very clearly stating that it's not ALL, mmkay?). Nor are they saying Wicca is bad, but rather pointing out that believing in witches as evil entities gets a lot of people tortured and killed.
And to be fair, there are a few cases that are tangential. I'm not sure that it's fair to blame feng shui in and of itself when somoene goes psycho and stabs someone over bad feng shui. I mean, I think feng shui is entirely laughable but in that case I'd blame the psychotic person, not the particular trigger (kinda like that current news story).
Anyway...if you've ever wondered "What's the harm" in letting some people believe anti-scientific things, go check out the harm. A lot of it boils down to bad decisions made because of quackery leading to injury or death.