Do teachers necessarily need a qualification?

First of all, I will just set out that this post is not an endorsement of Charter Schools. What it is, is a discussion about whether it is necessary for all teachers to hold a teaching qualification. I raise this in light of the comments made by John Banks in respect of his Charter School proposal and after consideration of a twitter thread I was reading only moments ago.
One reason for teachers requiring certification is that they are taught the ‘art’ of teaching, how to manage a classroom, how the curriculum works, how to assess work and so on. I suspect the teaching qualification does a lot more than what I have specified and my point is not to ruffle the feathers of teachers or to degrade the qualifications teachers out there have attained.  However, my argument is that having a teaching qualification does not equate to a person being any good at teaching.
One benefit for opening up the education system to non-certificated teachers is the availability of a broader spectrum of subjects thereby improving the quality of a child’s learning through exposure to more disciplines rather than the stock standard subjects our system is currently limited to (unless your in a private school).
I read a legal case and in which a judge determined that an ‘expert’ for the purposes of giving expert evidence in court did not require a formal qualification in his or her field. I can’t recall the name of the case, but it involved a Kaumatua in respect of a Maori customary fishing  and the statement made by the judge was that a person steeped in the lore of his people was a scholar by any standards and he rejected a formal qualification as determinative of expertise and considered such a requirement eurocentric. The Evidence Act 2006 also says that an  expert means a person who has specialised knowledge or skill based on training, study, or experience. Arguably, expertise is equally as good an indicator of the credibility of a teacher as a generic qualification.
If we apply the ‘expert’ argument to teaching, then surely we can accept that in at least some subjects, a teaching qualification is unnecessary provided the unqualified teacher is familiarised with the national standards framework. I’m not proposing that an expert in a particular field will be a better teacher than a qualified teacher, and besides, who or what makes a good teacher is really subjective and defining ‘good’ in this context is probably complex also. 
I’ve seen arguments raised about whether or not an unqualified electrician should be allowed to rewire a persons house, or whether a lawyer should be allowed to advocate in court without a practicing certificate. I’m not convinced that these are good analogies. Almost everyone engages in some kind of teaching in their lifetime and the art is a skill learnt over time. parents teach their children, Managers teach their staff, staff teach other staff, students teach other students, sports coaches teach sports teams, sports people often teach other sports people, the list goes on. My point is that teaching is a broad discipline with multiple sub-disciplines where specialisation may or may not occur, conversely, trades are specialised in their own right. The benefit of having a teaching qualification is that you are qualified to teach across disciplines, whereas if you are a non-certificated teacher you’d be limited to your area of expertise.
So my conclusion is that teaching qualifications should not be disposed of because there are greater benefits for persons who hold such a qualification; however, I do not think it should be a prerequisite that a person holds a practising certificate in order to teach provided they have the requisite skills, knowledge or experience to teach in their respective discipline.