The way to the Spiral Staircase is only found once a Search for Secret Doors action is taken. It's entirely possible a Hero would continue exploring through the door at the end of the hall that leads to the Spirit Riders rather than discover the hidden way out of the Quest. And then there are those who explore everything like drathe's groups have.
It's even possible for Zargon to 'encourage' a wiser Hero to abandon the corridor before a search was possible. Due to the random nature of the Revolving Room, a Hero might first need to face the 3 Zombies in the corridor alone, and that could sufficiently weaken him. Meanwhile, if there was an open path for a couple of Chaos Warriors or Mummies from the other rooms to reach him, Zargon could try to force him to retreat through the door that leads to the Spirit Riders. Monsters don't trigger traps, so Zargon is within his rights to judge that his monsters could ignore the random effect of the Revolving Room (Zargon ain't no boyscout).

To answer the OP's question, I think the purpose of the Spirit Riders is to provide fluff in an otherwise thematically-empty find-the-door Quest. That's enough to attract certain groups to the alluded tombs like moths to a flame. They're seeking out the journey more than the destination.
Wow, a mist that can cause Borin's Armor or Orc's Bane to crumble like some kind of Super-Rust spell.

It appears the designers of RotWL were retroactively seeking a way to keep the Heroes from maxing out too early. What will they think of next?
