On a more contemporary note, research has shown that if you replace an intrinsic rationale (e.g. patriotism) for an action with an extrinisc one (e.g. money), its impossible to go back. I imagine its a lot easier to create patriotism than money, so people want to stick with that solution.
That's debatable. Swiss mercenary pikemen enjoyed two centuries of well-earned respect for their fighting abilities, for instance. Or you can take the Varangian Guard of the Byzantine emperors, or the Gallowglass mercenaries. The issue was more often that many of them would not hesitate to change sides if it was seen as more profitable.
Though it should be said that employers were not necessarily more trustworthy, and could be prone to "forget" to pay the mercenaries they had hired once they had no more need for them.
Well at least that's not a concern in the case of ebola researchers.
We'd be less likely to send people into battle if we actually had to pay our soldiers what they're actually risking.