I don't think this is a waste of Hawaiian tax money.  Trump's campaign promises have begun an important debate about the core principles behind the articles that constitute our nation, and whether or not they can be interpreted in a divisive way.

The article you linked (thanks!) indicates that judges hold diverse opinions on the 9th circuit's decision on Muslim Ban 1 and that they stand ready to participate in the debate.  I don't see evidence for your contention that "most with a legal background" think the debate is a slam dunk.

Hawaii's argument against Muslim Ban 2 was produced with fewer time constraints and takes a different tack.    If I understand correctly, the government's challenge to Watson's decision was weak.  In addition, as the CB article points out, Trump's tough guy defense of Muslim Ban 2 appears to have strengthened Hawaii's argument.