Indeed, I assume such contractors are in action right now over Yemen:
The militaries of gulf nations have been “a combination of something between symbols of deterrence and national flying clubs,” said Richard L. Aboulafia, a defense analyst at the Teal Group. “Now they’re suddenly being used.”
The article is about how we've sold the aircraft for Saudi Arabia's coalition.
But I assume that mercenaries have a rather large role in flying the missions and probably almost all of the maintenance tasks to keep the planes in the air.
You can see my thoughts over the years on the issue of privatized military power (for only 99 cents!)
And don't forget that standing militaries can have bad side effects, too. Mercenaries are just a tool that can be used for good or bad purposes.