The leading case for executive privilege is United States v. Nixon. In that case recognized the privilege stating,
A President and those who assist him must be free to explore alternatives in the process of shaping policies and making decisions and to do so in a way many would be unwilling to express except privately. These are the considerations justifying a presumptive privilege for presidential communications. The privilege is fundamental to the operation of government and inextricably rooted in the separation of powers under the Constitution.
The Court found, however, that the privilege is not absolute. When the privilege conflicts with the rule of law the court must use a balancing test between the Presidents need for confidentiality and the “fair administration of criminal justice.” The importance of the executive privilege ranges from the extremely important area of national security, where the Privilege will almost always be up held, to the claim of the privilege for the sake of confidentiality itself, where the privilege is at its weakest. The strongest reason for abrogating executive privilege comes in the context of criminal trials as opposed to civil cases.
With this backdrop I believe the Presidents claim for executive privilege in this case is strong. Because this is a congressional hearing, the judicial process is not at issue. Furthermore, this is not a case for promoting secrecy for its on sake but rather secrecy in the execution of a constitutional power of the President. The President has the power to fire US Attorneys for any reason.
The purpose for which the information is being sought is also weak. This is not a criminal or civil judicial case, but a congressional hearing. It appears the real purpose is to simply embarrass the president. If every time a president took a legal action within the scope of his constitutional powers his aids could be called before Congress to testify about what advice they gave him, those aids would be more likely to hold back on what they say to the president. This concern squares exactly with the rational for which the court recognized the privilege.