Scott appeared to brush over the idea that you're asking a doctor to violate his oath and kill another human being. I think that's at the crux of the debate.
You have reasons for not wanting to kill another human being. Promoting "don't kill" to a terminal goal and forgetting that you have reasons for not wanting to kill, and therefore that you should be willing to kill when those reasons don't apply, is a lost purpose. (I'd link you to the post here about lost purposes but it is horrible.)
Also, in this specific case, the doctor has a legal monopoly. Getting and administering euthanasia drugs without a doctor is illegal, even where physician-assisted suicide is legal. If you want the doctor to be able to opt out of killing, then you should also remove the legal monopoly which forces the patient to use a doctor in order to be killed.
Getting and administering euthanasia drugs without a doctor is illegal, even where physician-assisted suicide is legal.
This isn't specific to euthanasia but applies to all prescription drugs. As long as the law doesn't let people decide to take even minor mostly-harmless medications without a doctor's approval, it will certainly not make an exception for drugs designed to kill people. Changing these laws is far harder yet than changing the laws prohibiting suicide and (non-medical) assisted suicide in general.
Over at Scott Adams' Blog you can find a very fine example of using the 'Rationality Engine' to solve the social problem of assisted dying.