I've opted in - my perspective as a social worker working with people who are aged, have cognitive disabilities, or multiple chronic health conditions is that the current system is incredibly disjointed and important things get missed all the time.
I cannot count the number of times I've seen medical professionals ask someone with a cognitive impairment questions about their medical history that they will then base their treatment on. The patient often doesn't remember and frequently gives wrong information. That is not quality care.
For my personal situation there are a few small things that will be useful such as having a list of my immunisations but I don't think it will be a big deal until something goes really wrong health wise.
Some other benefits of a central record might be:
1) Managing 'doctor shopping', and also making it easier for people who genuinely need schedule 8 drugs to get a prescription wherever they are.
2) Taking the pressure off parents and carers to remember everything about their loved one, and making it more feasible for someone other than the primary carer to support someone at medical appointments.
3) Reducing the need for people with long and complex medical histories to explain then to every new professional they see.
Looking through my MHR there does seem to be a lot of control over giving and restricting access to particular professionals, and the ability to review everyone who has accessed your record, but I am not sure how well this works
I fully support people opting out and raising hell about the privacy concerns though, it will make the system better.