S'mon said:
I kinda agree with this. I don't think much of the Song of Roland (Frankish posturing glorifying an ignominious retreat) but there are plenty of legends where individual 'high-level' heroes are the equal of several hundred 'mooks' - Arthur is said to have personally slain over 900 Saxons at Badon Hill, for instance. Unless the mooks are fanatics, they won't line up to be slaughtered, though. IMC on a typical large-scale battlefield, over the course of several hours a Fighter PC will have to take a move action to attack an NPC, and will rarely get to Full Attack and Great Cleave, so he'll typically be killing 1 man/round (and I start requiring Endurance checks after 50 rounds of combat).
Uhgh, that sounds horribly tedious.
I think you misunderstand the point of such uber-encounters. They're not there to give an opportunity to beat the PCs to within an inch of their lives or live out the experience of personally killing 5,000 people with a sharp (or blunt) stick. If I wanted to put the PCs in mortal fear of their lives, a few uber-monsters is a tool much better suited to the task than an army.
In FotR, the orcs all charge straight up to Aragorn to be scattered like ninepins, even if the smart thing might be to stand back and fill him with arrows. Ditto at Helm's Deep. In _Hero_, the emperor's guards keep charging the two warriors who have stormed the palace, even as their comrades are lying in heaps on the ground. The reason for this is because it demonstrates to the theater audience, in the most graphic terms, how the heroes are a force to be reckoned with. In an RPG you don't have a theater, but you still do have an audience of sorts, namely everyone around the table.
Nor would I actually go to the trouble of playing out each and every meeting between a 1HD orc and a 20th level barbarian. I'd probably just say that a pack of a dozen orcs charges the barb, and have the entire encounter played out in a few die rolls. At most, the orcs might hit once (if they roll a natural 20) and do a nominal amount of damage. Repeat as necessary.
This clearly isn't an approach that emphasises verisimilitude above all else, but a stand-up fight between an army and a few individuals is highly unrealistic to start with. Trying to inject some degree of realism isn't going to make that go away. To me, the point of these fights is to provide spectacular action sequences that showcase just how powerful a high-level character is. And that means playing the monsters in a way that plays to the characters' _strengths_, not exploiting their weaknesses. If you prefer more realism or grittiness, then high-level D&D (which is basically a supers or wuxia game in disguise) basically isn't for you.
Does this mean the entire fight should be a shoo-in? Of course not. The mooks might be just speedbumps, but every army has an elite core. In FotR, for example, you knew Lurtz was the named enemy, because he had his own backing music and a slo-mo introduction.

You don't have these tricks in an RPG, but the same principle applies. After the PCs have had their fill of slaughtering mooks, they can meet the _real_ opposition, who will then proceed to show them what a real fight is like. This has the bonus of providing a dramatic climax for the entire battle; an encounter that stands out compared to what came before, and so remains fresh in the mind afterward. Because really, after you've killed 1,000 1HD orcs, the next 500 will probably all look the same.
A platoon of NPC elite fighters might choose to engage and melee a 20th level PC fighter, but most grunt NPC melee troops will flee the F20's immediate area rather than fight a hopeless battle. The big threat is from companies of missile troops - 200 archers pumping out 200 arrows/round averages 10 hits on the PC fighter, for around 50 damage/round (assuming d8 with the occasional crit), so such formations need to be dealt with ASAP.
Um... why? What exactly does this prove, except that as a DM, you can hose the players any time you want?
Edit: The result is, IMC a 20th level party might well take on an army of 3000 goblins in a pitched battle with a reasonable hope of success. They'd be foolish to take on the 200,000 strong Mongali Horde, though. Extravagance works both ways.
If you have players who want to take on the 200,000 strong Mongali horde, more power to them, I say. Unless you don't want the burden of working out the consequences of such a major change to the world, which is a perfectly fine reason, but is metagame rather than in-game. Or unless the player is an annoying git, in which case, I'd just kick them out.