Bill Poser of Language Log asks if the plain meaning of the [recess appointments] clause renders most recess appointments unconstitutional, why has this illegal practice been permitted to continue for so long and, apparently, without much controversy?

To me, this seems like another example of what Barnett calls the presumption of constitutionality. The wording of the clause has been construed to allow for the broadest amount of executive wiggle room, just as McCulloch v. Maryland construed the necessary and proper clause so as to give Congress broad leeway in determining the necessity and propriety of its own legislation.