I agree that the outcome may have little direct impact. However, the Court's decision may still be a big deal.

It's the reasoning the Supreme Court might use in ruling against Massachusetts that worry me.  For instance, in portions of its briefs, EPA essentially argues that it should be able to read global warming out of the Clean Air Act because the agency believes that regulating greenhouse gases could cost too much.  I'd hate to see an opinion out of the Supreme Court adopt this view.  It could provide all sorts of ammunition for those seeking to undermine environmental law.

I agree that whatever the result it will not force the administration to act, but there is great value in just having this before the Court.

Whatever the Court decides, it can and should be overridden by Congress, which should be moving on specific legislation that is more appropriate than the CAA.