Many other people have chimed in about how it isn't just insurance matters make it important to keep medical issues private. Here's an issue that's close to my heart: Asperger's/Autism. My son was diagnosed with Asperger's last year and, while I haven't been formally diagnosed (mainly because that would require spending money we don't have and wouldn't help my son or me), it's pretty clear I'm an Aspie too.
Now, towards the end of last year, there was that horrible shooting in Newtown. One of the first things that the press did was seize on one statement from the gunman's brother about how he thought the gunman had Asperger's. It was completely irrelevant to why he shot those people, but the media put them together because it made for a nice scare story. "Is that person with Asperger's next to you going to kill you? Tune in at 11 to find out!"
Needless to say, some panic erupted. People formed "kill Asperger's" groups on Facebook. One teen, known to have Asperger's, was assaulted in a store because he had his hands in his pockets. (He was using something that helps him stay calm in social situations. He didn't show any signs of having a weapon but they beat him up nonetheless.) Thankfully, the Asperger's/Autism community reacted quickly to get the message out that Asperger's had NOTHING to do with the shooting. Still, some people still think that the Newtown shooter did what he did because he was on the Autism Spectrum. Similar acts of violence/prejudice have happened in the past (the 80's and AIDS) and will likely happen in the future.
Now, my wife and I are pretty open about our son having Asperger's, but I don't want that to be public information. I don't want someone to go to MedicalMaps.Google.Com, search for "Asperger's" and get a map with markers - one of which is my son and where he lives. This should be up to my wife, my son, and I who we disclose it to. Not some big company who sees my son's (and my) medical information as a pathway to profit.