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.