Your medical information is incredible sensitive to you and others around. With it you can be attacked, discriminated against, blackmailed and not to mention identity theft.
Besides discrimination. How?
How can you really steal my identity with my Health information? Remember Social Security Number is not Health information and Date of Birth and Name is part of the Public Record.
How can I be Attacked by my Health Information? Unless you are going through the trouble of tailoring something specifically for me. If you go through that much cost and trouble you could get that information anyways.
Blackmail in my case would not work. I've been part of interviews for Security Clearance before. One of the interesting things I learned is your sexual orientation didn't matter but, HIDING your sexual orientation could cost you clearance because, that could be used to blackmail you.
Discrimination I even addressed some. Though that can happen for many reasons.
In the end I think the real reason is that we do not like someone else knowing something we consider base to ourselves and exclusive. The usefulness of the information is not overly relevant its the idea of knowing its mine and only the ones I choose know it.
I have more to say but that broaches straight into politics.
Suffice it to say that I be more upset if someone shared naked pictures of me on the Internet because of my low self image then I would be if my straight medical history where out their. Now the day we start using straight biometrics for more things my opinion will probably change. Though with an always on Camera the whole naked picture thing would be vastly more likely.