As dennydpoints out, even though Charles Clarke admits ID cards wouldn't stop attacks, they're still planning on being introduced because... er, well, we don't know. They probably wouldn't help in the aftermath investigations. Actually, we're thinking we could just use them to introduce infringements on civil liberties in the name of public safety. Great. I think this is what people have been afraid of.
At least he admits that he wouldn't want to live a life of security checks etc.