A Brit can pass a RTW check without a drivers license or a passport - a paper birth certificate is also acceptable (and paper can be lost, damaged, forged), as neither a drivers license or a passport a mandatory. Getting those can be expensive for some people while this ID is free.
A NI number is not ID, it's a reporting number.
Lastly, a national ID is a tried and tested scheme in many, many countries and brings a lot of positives. The only "negatives" are slippery slope make-believe scenarios not based in reality.
https://www.gov.uk/prove-right-to-work
