I believe that ultra vires in courts is often referred to as "abuse of power by courts."
I think that this term, ultra vires, has far greater importance than we realize. My view is that it was established to put a check and balances on local, state and federal government powers.
A higher court, for example, can determine that a lower court's decision is ultra vires and invalid. But the same applies to the highest court in the U.S.- the Supreme Court. The Supreme Court has very specific powers as well and it can also make decisions that are ultra vires.
I think ultra vires all comes down to making sure that each institution in the US- whether it is local, state or federal- keeps within boundaries and only practices the powers that it has been given.
This is another reason that makes me proud of the system we have.