Shortstripper,
I understand many PPLers have the skill set, and relevent experience to make good instructors. There are also those (myself included) who spent more time looking at my 'confuser' than I did at my text books during my PPL course.
The requirement to complete the CPL groundschool course does unfortunatly stop some 'would be good' instructors teaching. But it also ensures a basic minimum technical knowledge, above that of PPL level, so when students ask the slightly more in depth questions the instructors level of knowledge is above that found in the PPL text books.
In my opinion the CAA have got this one right, the CPL ground course (although weighted to commercial operations) is a good way of improving on the basic knowledge taught at PPL and private flying level. The FI course will provide the flying standards and teaching skills required to instruct.
SW.