The D.C. Circuit (unlike the 3rd Circuit, which said resolving the issue was not necessary to decide the case) also noted what seems plain from the language of the Recess Appointment Clause: The vacancies filled during a recess must "happen during the recess," a requirement that makes Obama's appointments doubly illegitimate. The modern practice of waiting to fill vacancies until the Senate adjourns is plainly designed to evade the Constitution's advice-and-consent rule.
Such shenanigans are a relatively recent development. With the exception of Andrew Johnson, whose controversial appointments played a role in his impeachment, all presidents limited their recess appointments to breaks between sessions until Warren Harding in 1921. Intrasession recess appointments remained fairly rare until the Reagan administration, during which there were 73, followed by 37 under George H.W. Bush, 53 under Bill Clinton, 141 under George W. Bush and 26 so far under Obama.
While Obama's number is less impressive than those of his Republican predecessors, he has broken new ground by arguing that he can make recess appointments whenever the Senate is not open for business. The implication is that the president can unilaterally appoint officials anytime the Senate adjourns for a holiday break, the weekend, the night or even lunch.
Modern presidents have transformed a clause aimed at allowing them to fill posts when the Senate can't approve their choices into a tool for filling posts when the Senate won't approve their choices. Obama's especially brazen use of that tool may ultimately discredit it forever, in which case he will have inadvertently helped restore the checks and balances designed by the Framers.