To the comment above that some standard treatments for the greater many could lead to death: I'd rather have a laminated card with me at all times that states my condition in plain English than rely on some bloke's ability to access my medical record online.
What would be the advantage not to opt out by the way? There's got to be some kind of return/reward for giving some privacy away in my view. Otherwise I'd only see the downside that some could use my records against me in the future.