There are examples where the patient/service user/'expert by experience' voice has been important. Where things go wrong, it's often the patients and carers who bring the issue up first, and often have to struggle to get their voices heard. I'm thinking about thalidomide, contaminated blood and transvaginal mesh inserts, to name but three examples. It takes a determined effort to break through the 'safe and effective' assurances. Not just healthcare either. Post Office accounting systems spring to mind too.
Having said all that, I'd still rather trust a systematic review of a bunch of decent randomised controlled trials over and above a handful of people with pronouns in their email signatures participating in a focus group. If there was a Cochrane Collaboration appreciation society, I think I'd join it at this point.