I certainly think SAS has a very valid point. The sheer volume of information in the ATPL, CPL/IR groundschool is quite astonishing. It took me 2 years to assimilate enough of it to pass the exams. A lot of information is utterly irrelevant to the PPL but a lot of stuff even I have to admit has come in very handy at the most obscure of moments. I have found the same from 20 years of training diving instructors and examiners.
I am doing my Instructor course (CRI) shortly and am busy revising again to ensure I have an indepth knowledge of the stuff aspplicable to be a good instructor.
I think a good instructor who is one who has a rounded knowledge of aviation. You will often come accross the inquisative mind who wants to know more than just the PPL sylabus.