1. For the JAR-FCL PPL, the PIC requirement is 6 hours in the second 12 month period. For the old PPL it was 5 hrs in 13 months - the NPPL total was agreed by all as a better compromise since 2 x 5 x 24/26 = 9.23, so 8 PIC and a mandatory total of 1 hour of training flying comes pretty close. It also means that those who are less well off do not have to cram all their flying into the second 12 month period as they do with JAR-FCL.
2. The JAR-FCL training flight requirement is intended to protect passengers from the risk of poor pilot flying skills, as is the 90 day recency requirement. But the microlight community has a high number of single seat aircraft and reasonably argued against compulsory training flights for such people. This principle was then reasonably extended to other aircraft such as Turbulents, Pawnees and Cosmic Winds.
3. Accumulating training flying will allow more flexible training -such as an out and return to a grass strip, for example. Also a single hour in some SSEA or Microlight aircraft might be somewhat taxing. It also provides an opportunity to stagger the training over a couple of years if desired.
4. The extremes of some corners of the Class Ratings make it reasonable for at least some minimum to be conducted in each Class. But not 12 hours! Anyway, the industry experts all agreed that it was entirely reasonable.
Please do NOT compare NPPL and JAR-FCL, that is irrelevant in this context - compare the 'old' and 'new' NPPL requirements.