To me it's a question of benefits vs drawbacks. In my understanding ID cards are beneficial for
1) Running government services
2) Fraud prevention
3) Some immigration control (as at least some reason why people try to migrate to UK is lax document checks)
Some people argue that it's somehow becomes authoritarian, when you have an ID card. I personally don't buy that as most of us have already passports, NI numbers etc, so all the security cervices if need be have access to that.
Obviously if people start to engage with hypotheticals, that these cards will be used to check whether you can access internet etc, I agree, that would be dangerous, but that is not being proposed.
Also the argument that implementing it will take a lot of money and will never be done is not a convincing one, as in that case one should not really try to do anything in this country.