Thanks to Mark at Zenpundit, I'm not getting back to what I need to... why? Because I just read his post suggesting a read of Shloky's Private Militaries and Market States. A few brief comments... I apologize in advance for any rambling likely to appear...
To begin with, don't mislead yourself in thinking we're still dealing with a nation-state and all the baggage it entails, er, implies. The unit of discussion here is the state. The real customer of PMCs is not the tax payers but the state itself, or rather the elite of the state because they are the state. The people were brought in as cheap labor 150 years ago, facilitated by technology and nationalism, to fight industrial-style war. Nationalism, in the states most heavily using private military force (US and UK), is on the downswing. How many citizen-soldiers are really in either state's military? Look at the recruiting literature and videos for the US Army, US Navy, and US Air Force... look around and ask how many of your friends actually know somebody in uniform. Ultimately, how well can the state's leadership mobilize the people to fight its wars?
Accountability is a red-herring. That's a small advantage. Special Forces provide deniable accountability. The real question is what really opened the door to the use of PMCs? It isn't cost either because they are not less expensive in the long run, especially not in the Long War. How many recruiting cycles have we gone through now? How much more does it cost to raise 10,000 men for the US Army? over 50% more from 2001 - 2006, an increase that speaks volumes.
The PMC industry will adjust to the market, that's for sure. Some companies are, how you might say, very responsible, while others aren't.
How would you, Shloky or somebody else, contextual Israeli private military companies operating in Colombia or Mexico in this nation-state vs market-state discussion? How does violence become a commodity? Has it ever been a commodity (serious question)?