That cannot be the right answer. A court’s holding defines the scope of its power; holdings must be obeyed, by citizens and by other (lower) courts. Dicta is the stuff that doesn’t have to be obeyed. Saying “just look at how the Court itself defined its holding” is like saying: “Just let Congress decide on the scope of its powers.” Courts cannot be allowed to define the scope of their own power because if they are, they’ll do what all institutions do when allowed to define the scope of its own power: expand it unmercifully. Of course Roberts and the 4 Justices who are with him on this question would like it to be called a “holding”! They think they’re right, and they’d like to have their view on the matter obeyed by others. But the holding/dictum distinction prevents them from doing that, over and over and over again. Courts don’t have to be obeyed when they propound on something they didn’t have to propound upon for the purpose of deciding the case the way they decided it. To decide that the mandate is within Congress’ taxing power, they didn’t have to decide that it is not within its Commerce Clause power.
If there’s a “mess” here, it’s a mess that Roberts created by saying “My discussion of the Commerce Clause is a holding of the Court” when it clearly isn’t one.
[Tangential note: I owe this argument entirely, as alert readers will note, to Hamilton and Madison, in the Federalist. I had the truly extraordinary experience during the last 6 weeks of working through the Federalist Papers, one by one, for the class I was teaching at Temple's Rome campus (on The Roman Republic and the Constitution). I'll have a lot more to say about that here on the VC - I'm still processing the many things I learned from having done that. But one thing that reading the Federalist does, it makes you focus on a very basic principle: No man shall be judge in his own cause. Publius repeats it over and over again. Each of the three departments will be filled with "ambitious" people, all trying to aggrandize their own power, and the system is set up (in many ingenious ways) so that they can't do that]