Welcome to the forum.
I've read both of your TA related blog posts with interest and while I agree with some of the points you raise I feel I must join the debate - hopefully in a rational way
Like any other profession there are good and bad teachers with various levels of experience and expertise. Many TAs would, indeed, make good teachers and are, in fact, teachers in all but name (and pay packet).
Provided suitably trained and qualified I see no real reason why a school should not employ, for example, a SEN TA - an expert in his or her own field to complement the role of the class teacher.
Again, provided suitably trained and qualified, I see no reason why TAs should not be used to cover PPA with non-curricular activites. Many TAs I have worked with have known far more about art, for example, than I do - and if they're willing and competent why not utilise that expertise?
Teachers are not, and should not be expected to be, experts in all aspects of education - instead I believe they should consider themselves the manager of their classroom. As such the teacher is responsible for setting the overall direction of his or her class and the management / deployment of available resources (including teaching assistants / outside professionals).
Please feel free to call me cynical - but I believe one of the reasons for the upsurge in numbers of teaching assistants and other support staff is one of simple finances. Why employ one specialist 'teacher' when you can employ three specialist 'teaching assistants' for the same money?
The example you cite of the cover supervisor with no English or Maths qualifications is truly awful - all staff employed in school should be suitably qualified and have access to appropriate training - lest we return to the days of the 'mum's army' (who do have their uses - but not relevant to this discussion).
In your most recent blog post you state that 'I’m sure that in many schools teachers prepare detailed lessons for well qualified teaching assistants' - from my own experiences I'd have to say that's absolute hogwash (unless it's OFSTED time again) - while I'm sure there are some teachers who produce detailed lessons & lesson plans for their TAs I'm sure that they're very much in the minority.
As to some of the points you raise concerning the deployment of TAs in other countries, they are used on the continent - although not at the level we see in the UK - usually within a specialism. The role is also widely used within the US.
Your final analogy seems flawed. It is improper of you to compare 'Fred down the road who knows a bit about electrics & the electrician' to a properly qualified and trained SEN TA to a general classroom teacher.