Twiggles,
Schools are commented on by those who use them - that's what ParentView and parent questionnaires are about. Every school I have ever been in issues a parent questionnaire annually. Equally, I have commented on my experience as a patient at my Doctor's surgery (overall) and at my bank (overall).
However, where this differs from what you propose is that it is at an institutional, not an individual, level. And also that I am grading 'my experience', not 'my doctor' - in the same way that when I fill in my parental questionnaire for my children's schools, I am commenting on my children's experience of school (which I am absolutely qualified to do) but not on 'the ability of an individual teacher' (which I am not qualified to do, even as a fellow professional, because I do not observe them on a day to day basis, scrutinise their marking, look at a cross-section of pupil's books, look at progress across the class statistically, experience their subject co-ordination responsibilities, view a cross-section of lessons, scrutinise planning etc).
If parents are to grade teachers, then they would need to be able to do all of those things, exactly as those such as SMT and inspectors who do scrutinise teacher performance do, otherwise their view of a teacher's perfomance is partial and potentially biased too much by an individual viewpoint. Is that what you are proposing? Groups of parents observing lessons, scrutinising books etc etc?
If a parent has an issue with my teaching of their child or their child's progress, my door is always open to them. I spend many, many hours in such discussions, both positive and negative. Having just moved jobs, I know from dozens and dozens of written comments that I am held in high esteem and would be graded well. I still do not think that 'being graded by parents' would add one jot to the actual performance assessment or professional development of teachers.