[RichardCoulter]

All I can think of is that it sometimes poses the difficulty for some in proving that the NI number is theirs.

These days people who claim benefits are required to prove their identity and that their NINO actually belongs to them. There is a lot of NINO fraud in the system, particularly as there are more NINO's in circulation than there are people alive and living in the UK.

The most common ways to steal a NINO are to assume the identity of a child who died at birth/at a young age or the identity of someone who emigrated abroad.

Getting a job with a false NINO is much easier to do and is how some fraudsters manage to both work and sign on.