Greetings!
Yep...You really nailed it down Green Knight!

I suppose I run a very unique style of campaign, but since I have several huge empires in the campaign area, who all have vast armies, it changes the equation dramatically. In many ways, my world takes a lot of the assumptions of 14th century European D&D and throws them out the window!

For example:
The armies you speak of Green Knight--exactly! In many areas, soldiers in my campaign are very professional, with fine-quality equipment, constant training, and good leadership. From a leadership standpoint, the adventurers have several advantages:
(1) The adventurers are primarily expendable. They are relatively cheap, and they are anonymous. That means that the king can give them free reign to do "whatever needs to be done" and have no worries. Next, if the adventurers fail, there is no public-relations outcry, or groups of angry women marching in the streets holding candle-light vigils. That doesn't usually happen when soldiers die, of course, but it *can happen*, and the king usually likes to maintain his excellent, fatherly reputation as much as possible. With adventurers dying, it's no real loss. Just secretly recruit some more, until they succeed in whatever strange mission, that way you preserve the armies for really important struggles, and at the same time, the risk is next to nothing with adventurers. So, it's a good deal all the way around for the throne!
(2) Adventurers usually have a few more obscure skills and abilities thatn the average soldier. This makes the adventurer, by degree, more suitable for special warfare operations than the average soldier.
For example, a group of soldiers isn't likely to have all of the varied abilities to infiltrate a Dragon's enchanted, trapped, cavern complex, defeat all of it's minions, summoned creatures and so on, defeat it's deadly series of wicked traps, and succeed in killing the Dragon. No, the adventurers will clearly be in their element here. However, soldiers fighting adventurers is *not* the same proposition as soldiers fighting a Dragon.
Point: The soldiers in many cases have most of the skills that adventurers have. Most, but not all. The soldiers are well-equipped, well-trained, and well-led. Even if the adventurers have more levels, I can gaurantee that a platoon of Legionnaires can give any party something to think about, providing that the Legionnaires are aware of the adventurers presence. For example:
Vallorean Platoon--Legionnaires
(3) Infantry Squads, each composed of the following:
(1) Sergeant; Usually 8th-12th lvl Fighter/Ranger
(12) Legionnaires; 4th lvl Fighter/3rd lvl Feat Master is typical.
basicly they are 4th lvl Fighters with a few more hit points and a few more feats that better represent their abilities as professional soldiers of the Vallorean Empire.
(1) Weapons Squad, each composed of the following:
(1) Sergeant, usually a Wizard that is 8th-12th lvl.
(2) Bards, 4th-8th lvl.
(4) Rogue/Wizards, usually between 2/2-5/5.
(4) Wizards, from 4th-8th lvl
(2) Fighter/Wizards, 2/2-5/5 in lvl.
Thus, these four squads forms a Legionnaire Platoon. In addition, there is the command unit, composed of the following:
(1) Platoon Commander: An officer of proven ability, this officer usually has 16 lvls, composed of Fighter 8/Feat Master 5/Ranger 1/Expert (Scholar) 2.
(1) Platoon Sergeant: usually 10th-14th lvl. Same as a line sergeant with additional levels.
(1) Warrior-Priest, usually a Fighter 2/Cleric 2--Fighter 5/Cleric 5.
(4) Fighter/Clerics, 1-2 lvls lower in experience.
(1) Wizard; This Wizard is usually 8th-12th lvl, and has a number of magic items, in addition to having a special horse that has Horseshoes of Speed, a Saddle of Horsemanship, as well as a Chaffron of the Mystic Steed, which transforms the mount and rider into an indistinct form of shifting, silvery motes of blurring light. The form gains a series of protections and benefits. The Wizard can move over land at fantastic speeds, and summon help or bring messages. Should other forces be in closer range, magical communications is easily maintained. Extra platoons and legionnairy forces can thus be coordinated against enemy forces.
All members of the platoon will have appropriate selections of magic, based on character treasure/goods-guidelines. In any event, these forces have at the minimum master-work weapons, armour, and equipment. They have access to additional potions, scrolls, arrows, and such, on a group level. They have plenty of the best mundane equipment, and can also have several packs of ferocious Dire Wolves, who have been trained for war, and provided with Dog Armour, and Spiked Dog Collar. Uniformly, these special wolves have Dog Armour and Spiked Dog Collars that are enchanted. They have in common a variety of magic and spell-resistances. Under specific circumstances, the wolves can also become Hasted, and boosted with other spells.
These forces are skilled, and familiar with excellent tactics and strategies. They operate together like a well-oiled machine. Adventurers are skilled, but they don't have a monopoly on levels, skills, magic, or special abilities. As can be seen by the detailed Vallorean Legionnaire Platoon, it can be imagined why hiring adventurers is quite popular with the nobles and powerful people of the Vallorean Empire.
Training, equipping, and maintaining powerful, professional military forces is very expensive. But the payoff is considerable, too. Like dominating the political, military, and economic destiny of a continent, and safeguarding a vast empire of over 100 Million people.
Semper Fidelis,
SHARK