I would just like to agree that instructors Should have CPL knowledge to teach PPL's.
Try asking an old timer PPl who fancies doing "a bit of instructing" about a drag curve or any fundemental POF subject and I bet the conversation will last about 15 seconds and then knowledgeable PPL person becomes a mumbling blagger.
Remember these people last studied technical PPL knowledge many years ago, probably to a syllabus that is probably not taught any more and havn't done any structured flying for ages prior to starting a course.
Top teach at a level you need to surpass your students level of knowledge and be able to answer the awkward questions. Thats why people with degrees teach high school students, would you wish to have your sons and daughters taught by 6th form graduates?
At least a newbie CPL/FI has had to demonstrate a good standard of accuracy and flying TWICE fairly recently to get where he's at.
There is a country mile between how seriously a CPL skills test and an FIR test are taken by examiners there certainly was with both of mine (Both CAAFU).
Just my £0.02 worth, that said, the old BCPL type scheme should be brought back and a training credit given for the IR after 6 or 700 hours, this would settle down the instructor market and get rid of this phenomena of people instructing until the 737 job turns up