I really feel that the other articles in this theme have missed the most important part: Morale. I’m not talking about when one decided to turn tail and flee a battle; I’m talking about why they show up in the first place.
Why do adventurers fight? Most commonly, they fight for money. Someone promises to pay them for the mission plus they plan to loot the enemy. Now to many people, fighting for money is cold blooded and requires a rather sociopathic or psychopathic personality. I won’t argue the mental case, but even fighting for money can be rather hot-blooded.
Why do soldiers fight? For patriotism. Because they know their cause is just. Because their enemy is threatening their homes, friends and families. Are they always right? Are the enemy always evil? Nope. But the generals and rulers must make them believe that they are on the right side of history and they are the good vs. the enemy being evil. If the army’s leaders are unable to do this, the soldiers are going to be far less willing to go out and risk their lives in the taking of other people’s lives.
That is one of the chief issues between adventurers and soldiers, and from a role-playing stand point, it is of the utmost importance. An adventurer or other mercenary can weigh the odds once the battle is about to begin and decide if the cost-benefit here is worthwhile. Is it worth getting into this particular battle or should we just retreat? But the soldier has to weigh other factors: If I flee, will that mean the death of my family? Will people lose their homes to these savage invaders? Recent surveys have shown that perhaps one of the most important factors that they weigh is the question, Will my friends in the unit be killed or maimed if I am not there to help them.
In many ways, the difference between adventurers and soldiers is one of cold math vs. emotion. Adventurers can afford to reduce it all down to how many coins are involved, but the soldiers cannot afford to do that (no pun intended).