It is interesting that almost all of the comments in this thread concern themselves with the effect on instructors if a separate NPPL instructor rating were introduced. What about the effect on students and FTOs? I am not concerned about the quality of instruction from an NPPL instructor since a suitably structured NPPL FIC course could make that a non-issue.
I am concerned about the effects on a student who initially embarks on a NPPL course taught by an NPPL instructor and subsequently decides to obtain a JAR-PPL. How would any FTO explain to the student that the hours which he has flown count for nothing towards a JAR-PPL since all of the required instruction for the JAR-PPL has to be conducted by a JAR qualified instructor?
As a CFI I would dread having to tell a student that the hours he flew with Instructor 1 (an NPPL instructor) do not count towards his JAR-PPL but had he flown with instructor 2 (a JAR instructor) he would have been taught exactly the same things in the same aircraft at the same cost but the hours, even although the student had originally intended gaining a NPPL, would count towards his JAR-PPL. Do you wish to have that conversation?