I would turn the question that many above have asked on its head; if you have no wish to fly more than 3 pax, IMC or overseas then why would you go for the PPL?
The money you save on the medical pays for your biannual.
The NPPL is a lifetime licence.
You can easily add-on SLMG and/or microlight ratings.
You don't have to write a cheque to the CAA every few years.
If you're a day VFR pilot who is happy to fly in the UK only (or fly micolights abroad or ask for permission which seems to be fairly routinely granted) then why would you want to spend more money keeping your licence valid?
It may be small change to many but if you're in a group or flying a cheapish permit aircraft it can actually make a difference.
I'm all for the NPPL and I think it's a shame that the take-up hasn't been far wider than has so far been the case.