I'll probably not opt out but I might if I were younger, if half the world hadn't already got access to my medical records or (sorry, not my intention to alarm) if I had mental health issues. I don't care if employers have access to my physical health records, given the unsolicited approaches I get my details would appear to be well and truly circulated. But while it might help
medical practitioners to share information about my mental and emotional state, I would be concerned about any leakage elsewhere. That sort of info wouldn't be on offer until this scheme is well established and some evaluation available.