I believe this has been discussed in previous threads?
Allegedly, if you are an EU citizen, where carrying ID is not compulsory, Brussels felt that the same situation should be enjoyed by Brits. etc., living abroad.
When my NIE was due for renewal, I requested a further ID card but was told that as a British subject, I had to carry my passport and the green replacement sheet, showing my number and residency.
I feel that my pseudo-Spanish position has been diluted, even though I use an old NIE card for ID when I can, it is not quite the same.
Maybe one day, like the UK, we will have a choice?