Look at Morgellon's; Mercury Chelation; Anti-vaccination; etc etc etc.
There are very many people willing to sell tests, and very many people happy to sell quack cures based on those tests. (I'm not saying that 23andMe are quacks!)
> "Man, is there anything worse than being told you have something terrible wrong with you, living with that, then finding out it was a false alarm". Uhm, yeah. How about finding out it wasn't?
That's happened to a few people. You get told you're HIV+ (in the late 90s, when this means it's a death sentence.) You lose your job (because people are arseholes), you stop showing your 8 year old son affection (because you're scared of the infection), you have unprotected sex with people with HIV (you're already +, so what does it matter?) and then you get told that there was a mistake with the original test and you're actually negative.
That is a bad idea, even if you are 100% certain the result was correct.