1- We don't know where the actual nuclear weapons are being stored, produced, or developed. Given the fact that the NKs have moved their entire military infrastructure underground, it is unreasonable to assume that they would leave their most critical defense component totally exposed at a site that appears regulary on CNN.
2 - It forces their hand. Kim has two options, certain political collapse, or retaliation.
I'm all for keeping NKs feet a roasting, but we need a better end strategy before we stir up the hornets nest.
If it is impossible to take away their trump card in a surprise assault, then we are only left with the option of finding some way for them to compromise with us and give up their trump card. After their trump card has been so successful it is not within reason to think that they would surrender it in another agreement (as good as the last one?).
If things are as you opine, then we are essentially out of options other than buying them off (how much in taxes would we like to pay to buy off NK blackmail?)