That reasoning is probably a big part of why Obama is defending this in court. There is probably also a factor of not wanting to hand the Republicans an issue right before the elections. Obama seems like the sort who would rather hash this out in Congress then in the courts anyway.
However, I deeply suspect that Obama is being played here by certain people in Congress and at the Pentagon.. They know politics and Congress well enough to know they can simply delay things until there isn't time to deal with this under Obama. When a new administration comes in, they will either have a Republican one that will stay with DADT or they can repeat.
Jay