This came up (is the AG required to defend) several times when I was a Prosecutor. the law is settled that he does not. Further, any branch can decide if something is constitutional. While I absolutely 100% think they should vigorously defend this, through the History of the U.S. it has been just as clear they don’t have to.
I do not take Holder's statement as a prosecutor's discretion issue because he stated 'they do not have to defend laws against constitutional court challenges'. It appears to be his opinion that the law is unconstitutional and does not need to be defended. So the question for non lawyers is how do we have faith in the law?