I was thinking about this the other day, as my missus only works part time (about 20 hours per week) and with the new threshold, it's unlikely she'll pay NI.
From what I understand in the following link, if you earn between £123 and £242 per week, you don't pay NI, but you get the benefits of paying:
https://www.gov.uk/government/publications/rates-and-allowances-national-insurance-contributions/rates-and-allowances-national-insurance-contributions
I'm not 100% certain though, but that's how I interpret it .... would anyone else like to chip in??