It wasn't necessary for the court to decide on that basis so it didn't. It took the path of least resistance to make the clearest precedent.
It is, however, important they recognized the argument.
You go ahead and challenge Rubio or Jindal or how about Santorum or maybe you think Romney isn't a natural born citizen either.
The trouble with Birthers is you have no uniform agreement on a version of NBC: two citizen parents, two citizen parents themselves not naturalized, two citizen parents and born in country, etc...