I guess the remuneration issue is one, which will have a lot of self interest tied up with it. If you take the NPPL as a licence for fun flying, then we are talking about a hobby. You can find fully qualified instructors teaching most hobbies for free. Take Canoeing, mountaineering etc, all have qualified people giving free tuition in there spare time, I know, I did it for years.
If you allow these people to teach on permit aircraft from unlicensed strips you will be teaching people to fly light aircraft, who only want to fly light aircraft. Most current schools seem to teach you to fly a light aircraft like a heavy, because they may want to do an ATPL like the instructor teaching them. In this way we can separate out the potential airline pilot, who should pay for his training, his eventual aim is after all to make a living at flying, from the amateur enthusiast who has no interest in JAR upgrades and ATPL’s.