If we're going to be mildy realistic about military tactics for a moment (which is probably a mistake since Bioware is never realistic) the Wardens don't make sense as troops in a conventional war. They aren't knights for the most part - lore wise they're not supposed to fight on horseback like chevalier.
They're footsoldiers, archers and mages. That's not really a shock drop, and their numbers are too small to make sense in a pitched battle as a unit. This is what makes Duncan so terrible at his job.
Well, let's not be too uncharitable, because there are a number of assumptions here that might not be right.
For one thing, knights - and "well-armed, well-trained aristocratic warriors" generally - did often fight as heavy infantry quite consistently throughout the classical and medieval periods, from the
thorakitai of the Successor armies straight through Martel's iron wall at Poitiers to the English men-at-arms of Agincourt. These heavy formations were used quite profitably in defensive roles and offensive roles alike; they could hold a line, engage enemy infantry on even terms, and spearhead an assault - in fact, they often made excellent shock troops precisely because of their skill and survivability, and because (unlike heavy cavalry) heavy infantry could successfully engage formed enemy infantry.
For another thing, we don't really know how the Wardens at Ostagar were armed and armored anyway; all we know about them is that compared to the rest of the army they were described as being skilled warriors generally, and extremely skilled at killing darkspawn specifically.
We can't really rely on visual details from the game to rule anything out, because - presumably due to the limitations of the games' cinematics - mages were never shown fighting at Ostagar (even though we know that they were there) and cavalry of any kind was never shown until
Inquisition (and really, not even then, because nobody
fights from horseback in the Breach crisis), so we can't really say positively or negatively what battlefield role the Wardens were best suited to...and that's apart from all the other random silliness in all the depictions of mass warfare in the DA games that come from the limitations of the animation technology, the gameplay mechanics that emphasize small-unit engagements, and the Hollywoodified cinematics generally.
I agree that there's a disjoint between "stuff that can be done against darkspawn without Wardens" and "ostensible tactical value of Warden forces against darkspawn" in the games, but I think that's probably a result of other issues like "typical fantasy-fiction military writing". A two-hundred-year war against an infinite enemy that destroys crops and sickens the earth purely by being there is kind of silly on the face of it anyway, so drawing serious military conclusions from it is a fool's errand. But the mere fact that the Vints could hold their own against the spawn doesn't mean that the Wardens weren't better at it than they were - it just means that there were lots of other factors helping the Vints along (like massed battlemages, a world-spanning empire, and immense wealth). A comparison: the Entente powers' eventual victory in the First World War was achieved in spite of poor artillery fire tactics, lousy tactical organization, and lackluster operational sequencing, but the Entente powers possessed enough other advantages to bring the war successfully to a close. But,
had their armies employed Bruchmüller tactics or the Pulkowski method, or
had their armies pursued a more unified strategy and sequenced operations properly, they probably could've won the war faster, and with fewer casualties.
In another, more perilous scenario, the tactical advantage of possessing Wardens against darkspawn - supposing, again, that one actually exists - might be one of the few advantages left to a weaker Thedas facing a stronger foe than did classical Tevinter.
From a strategic standpoint, it's well and good to suggest that Wardens ought to be kept out of the fighting until an Archdemon appears, so that they can be hurled at it
en masse in a berserk effort to kill it at all costs. But then the doubts start creeping in. If Warden forces provide important tactical benefits, and if the armies of civilized Thedas can't defeat darkspawn forces without those tactical benefits, you run into a fairly serious problem. What if you can't catch the Archdemon? What if repeated darkspawn tactical successes lead to a point where even
when you finally know where the Archdemon is, you can't get the Wardens to it because the darkspawn horde is simply too strong, and Thedas is simply too weak?
It's similar to a classic dilemma in military thought: when to commit reserve forces to battle. Do you throw most of them in early to try to keep control of the flow of the situation, or do you keep them in hand until you spot the decisive moment? Both are highly contextual options. Spending reserves like penny-packets where they are unable to mass for the decisive moment
has sometimes failed quite dramatically;
other times, though, keeping the reserve out of battle until the decisive moment is futile because there never really
is a decisive moment, and the rest of the army is ground down until all the reserve can do is cover the inevitable retreat. If you know which of the two is better to the degree that you can declare so confidently what the Wardens should always do, I imagine Carlisle, Sandhurst, or the École de guerre would love to have you, because you've solved military theory.
More importantly, though, there's the point that in-universe, basically all
direct depictions of Wardens show them to be tactically valuable against darkspawn, or at least
expected to be tactically valuable in that role by everybody else. That's the role that they played in
Last Flight, and it's the role that the conscripted Wardens play in
Inquisition. Cullen, the closest thing to a military authority in any of the three games, directly states that he defers to Warden judgment on the topic of killing darkspawn, and says that that's what they're best at. While Blackwall was still thought of as a Warden, the Herald did the same thing. I don't think that there's any serious question that, at least for now, the writers intend for us to view Wardens as being better at fighting darkspawn than anybody else. Any other view would rely on a fairly hefty dose of error theory, and therefore headcanon.
Naturally, since this is a fictional enterprise, the writers reserve the right to change this depiction at any time.