Yesterday I wrote about how Obama could be facing his own version of Proposition 8 if he fails to repeal "don't ask, don't tell" while defending it in court. Like Gabe, my personal feeling is that he shouldn't be defending it.
That said, unlike DADT itself, the argument over whether the administration is obligated to appeal is one reasonable people can disagree on. I think it's perfectly valid to argue, as former Solicitor General Paul Clement has, that the president refusing to defend laws he disagrees with -- even those as blatantly unjust as DADT -- sets a problematic precedent. I don't like the idea of a Republican administration, for example, simply deciding that they no longer want to defend the Voting Rights Act because they think it's an unjust imposition of federal authority on the states. Still, Clement's predecessor, Ted Olson, has pointed out that this has occurred before without the kind of blanket chaos Clement fears.
I don't understand why the administration, if it feels obligated to take Clement's position, hasn't employed the compromise offered by former Solicitor General Walter Dellinger, who suggested publicly declaring the law unconstitutional while continuing to appeal it. This would make the survival of the law less likely even in the event that repeal falters in Congress.
Obviously the administration would prefer to repeal the law through Congress, both as a matter of substance and politics, and it has made public political commitments to the military it doesn't want to break. But it seems to me that the administration should be carefully planning for the possibility that repeal might fail, which would leave the courts as the only option for the near term. If the policy undermines national security as drastically as the president says it does, I don't know why it hasn't taken all the steps it could possibly take to ensure DADT's demise while still complying with its own stated principles.