It makes me think of the quote attributed to Benjamin Franklin: "When in doubt, don't." I accept the argument of the executive branch refusing to enforce a law the president thinks is unconstitutional. I would hate to see it turned on its head, though, where this argument is wrongly used to all government action. Say the president thinks that people are being intimidated from exercising free speech or another guaranteed right, so she starts taking government action beyond current law to protect that right. In that case, this could become just another excuse for increased executive branch power.

Even if we say this concept only allows the gov't to refuse to act, not to act, I still think it should be used cautiously. In my mind, things fall into constitutional, unconstitutional, and debatable. So failing to enforce a law should only apply to things the president or other gov't official thinks are patently unconstitutional. Otherwise, I could see this being perverted into making the exec branch even more powerful.