3D mobility, strength, speed, unique powers such as poison or webbing, and so on. Just think of that combination of a fearsome drider with a cunning rider (like our paragon) as they execute their plans in situations like claiming Reeve Keep, for instance! And it's not hard to imagine that Rowan could turn this experience into developing retinues of "orc knights" with their drider mounts with the help of some brave and loyal soldiers, Draith, and others.
Drider mounts would be quite decent. As would Centaurs and similar.
Driders, if they can be made sentient would make for ideal skirmishing cavalry. If they can move side-to-side, they could avoid missile fire (So long as they can see it), and if they can jump somewhat like Jumping-spiders or Hunstman spiders can... then they could make leaps in surprise attacks. They would also be quite ideal if they are sentient in that if they are not used as a mount, they will be be virtually unhindered and thus should retain a great level of endurance and mobility (With perhaps the exception of barding).
Of another benefit to Driders and Centaurs is that they have free arms. This means that they can hurl projectiles, be they for throwing javelins or darts, but they could potentially use bows/crossbows. I'd also wager that Driders and Centaurs can throw javelins harder than humans can from horseback (The reason being that when Humans are seated, they can only use their hip movements to aid in a throw, whilst a Centaur or Drider can likely use their legs and then their hips) This may not sound like much, but it really adds up. (It can mean the difference between achieving throws of say, 40 metres with a hip movement vs 50-60 metres with a run-up +leg+Hip movement)
And when mounted, they might lose some of their initial endurance, but so long as the rider does not impede them very much, it would not surprise me if they can still use ranged weapons. So you essentially have a mount that has two, not one person on it. This means that you could potentially have multi-role cavalry.
On the downside, if Driders are sentient, they may be less inclined towards charging ranks of men from the front, if used in say a shock role. Even warhorses tend to avoid charging towards large frontages, they are smart enough to know that spears hurt them, a lot, the riders not so much.
As to Orcs, I don't really see them performing well in a 'knightly' role (I.E armoured Shock cavalry). Light cavalry duty is probably more on the level they would perform at. Raiding, attacking routing troops, sending messages, harrasing the baggage trains etcetera. They likely won't be armoured enough to perform well in other duties, such as attacking the enemy directly (It seems to me that they tend to shun the wearing of armour, which is somewhat explored with why Eidood won a matchup after talking with Rowan). The difference between skin and maile is significant, plate armour even moreso. They won't be able to hold for very long if they have to fight against stationary troops of men. They also likely won't survive an engagement with more heavily armoured cavalry.
Even if Orcs are stronger and larger physically, bare skin ain't going to count for shit against a spear thrust. Depending on the size of the enemies that any given Orc raider could be facing... they could well be within range of two or three spears (Spears often being anywhere from 2-3.5~ metres in size) for said Orcish light horseman/driderman to be within attacking range of one human soldier on the front. Even against a more looser spacing of men, its still about one man per metre, so an orc raider being within attacking distance of two spearmen seems quite valid to me. You want that armour. Even a shield would be a great net-positive. On the other end of the spectrum, If some Orcish raider was dumb enough to attack a pick-block from the front... the poor bastard would probably have to contend with about 20-or-so Pikes being in range before he can be within his weapon-range to attack a single pikeman.
"Oh, but Orcish skin might be more like leather!" You might be saying... well, I've seen what a spear thrust can do to leather. Even if Orcish skin was more akin to Rawhide, a spear thrust is still going to go through deep enough to do some damage. On that note, Underarm thrusts from a spear are often weaker than overarm thrusts. Two handed spear thrusts though, are fucking vicious. ~100+ Joules concentrated on a steel point thats probably ~0.1-0.2mm wide... not something I'd want to be hit with. Polearms are worse, not least because they tend to have more mass on the top than spears comparatively.
However, it should be said that if the enemy is thin enough, perhaps only ~4-5 men deep, Even with light cavalry you could likely push through that formation by a well-lead charge. This could split the enemy and allow your cavalry to get to work on attacking two greatly demoralised halves. I haven't had a horse charge at me before, and their's no way I have the budget for it... but I know a guy who knows a guy who was at a Reenactment (Viking) where he got charged by a horse. He got boweled over and he was flung about three metres back, It's Good thing he was bracing with a shield. If its thin enough, you can get through. But if it's not, it probably won't be fun for the light cavalry.
So Orcish light-cavalry attacking troops themselves... It's possible, but its a big risk.
A heavily armoured Orc on a Drider, probably with some form of barding... That would certainly be something to see. They could very well hold their own against almost anything. With perhaps the exception of charging a pike-block. Because fuck charging a Pike-block. They could reasonably lead charges against lesser troops, even from the front, though they will risk casualties in attacking the front.
Another role that Centaurs and Driders might be really great at on a battlefield? Inserting ranged troops to locations quickly whilst providing ranged support themselves. If, theoretically each Drider is armed with a ranged weapon, and their riders are armed with ranged weapons, then driders/centaurs could be ideal for quickly transporting ranged troops quickly where ranged support is needed, and then quickly moving on if needed 'on the fly'. You could also mix up the ranged weapons for differing purposes. Bow-equipped driders and Crossbow equipped Human mercenaries... or Crossbow equipped centaurs and hired shepherds armed with slings etcetera etcetera. Centaurs/Driders will also likely be useful for carrying a fair amount of ammunition in the given engagement.
And, if Driders can make enough sticky webs, why not use them to set up the pre-battlefield environment with traps. Sticky webs to slow movement, perhaps with a couple thrown caltrops... boy, that would sure fuck with peoples day.