The issue is one of scale. A few individuals or a "security service" is not quite the same thing as entire regiments for hire. Indeed at the height of the renaissance entire armies could be composed of hodgepodge of merc troops. Things got real nasty when they didnt get paid.
Large scale merc operations basically died down after the Napoleonic wars.
Rat
I'd say 100,000+ private contractors in Iraq constituted a large scale merc operation, even if the majority of those folks weren't involved in combat operations (and would object strenuously to being called mercs).
http://www.washingtonpost.com/wp-dyn/content/article/2006/12/04/AR2006120401311.html
From Civ5, I have learned...
1. If you don't understand economics, you can't build a windmill. Here I thought it was the wind making it turn, when it was actually Adam Smith's invisible hand.
2. El Dorado is real, and sprang forth from the earth spontaneously.
3. All trade that happens in New York City occurs because Route 95 passes through Washington, DC.
4. The residents of Washington, DC have steadfastly refused to have anything to do with railroads, except for travel to other cities.
5. All granaries are exactly the same size - and they are always full, even if a city has no farms nearby.
6. People suffering under military occupation are much happier when your lawyers show up.
7. If two countries agree to research something, they don't actually work together. Scientists from one country will apparently only accept monetary funding from their government if another country's government is also funding their own scientists, even though their projects are not related.