I am a 23andme member and my genetic test showed that I am sensitive to warfarin. That's something I never knew before. If I ever get into a situation where that drug is used, having informed the doctor of this potential problem just might have saved my life. There is no possibility that this information could result in any harm, because if the doctor gives a lower than normal dose and it's not effective, he can simply give more.
What if the test is incorrect? You claim that there's no risk because the doctor can just titrate the dosage up, but if you have a time sensitive condition then the extra time required to adjust the dosage up could result in irreparable harm to your health. E.g. you get a stroke or thrombosis while the dosage is being adjusted.
If I were going to use a test like this, I would use the test, then go talk to my primary care physician with the results along with my known, diagnosed family history and my own medical history before I need urgent care. That way we can look it over and decide what the best course of action is. Maybe this is saying "thanks captain obvious, two people in my family died from this, it was a known risk at birth." Maybe the answer is "this is plausible, it requires further testing using more accurate methods." Maybe the answer is "this is batshit insane, forget about it." Regardless, this is not taking the results at face value, but reviewing them carefully, one by one, with a trained physician along with my family and personal history.
That being said, half the time medicine is a crapshoot anyway. January 2013 I wound up in the ER due to a probable allergic reaction to Amoxicillin. I was suffering anaphylaxis, stopping short of the most severe symptom (throat closure) due to quick reaction and prompt administration of drugs. This could have been a one time occurrence, or it could have been a drug allergy that I developed all of a sudden. I spoke with a specialist who basically said "I can test you but it will cost money, is not very accurate, and in the end you're best off just not taking Amoxicillin and assuming you are allergic." So, meh. Even the "real" medical tests are not 100% accurate and worthwhile.