[Formendacil]

I think it's worth emphasising the symbolism of the names on the list. Even from the way Imrahil talks about it, I think it's clear that a big part of the reason Sauron will take notice of the names, specifically, of those marching on the Black Gate is because of what they stand for, that is, what they symbolise, not necessarily what actual force they command.

With regards to the Dúnedain of the North, they are a reminder that Sauron failed in Eriador. While Arthedain is no more, neither is Angmar; and of the two, Arthedain was the one that left behind survivors--left behind an institution in fact. Admittedly, the North-kingdom was no longer a military threat to Sauron's takeover of Middle-earth, but their continued survival was a sign that Sauron, just like the good guys, had been fighting "The Long Defeat" without complete success.

What is more, I agree with Uruk's Bane that Sauron probably knew about the existence of the Rangers. After all, they'd been thwarting his wolves and goblins in Eriador for centuries (take note of all the premature deaths noted among Aragorn's ancestors), and these incursions must have had at least some direction from Sauron during his time as Necromancer.

With that in mind, consider what Sauron must have thought when the Dúnedain of the North made their reappearance in the broader world in the immediate wake of his discovery that Isildur still had an Heir. Aragorn's appearance would have made the very existence of the Rangers go from being an irritation in the North to a symbolic slap in the face of three millennia of planning. If they were to march on the Morannon and all be slaughtered, Sauron would be able to destroy more than just an annoyance, he'd be able to effect a massive symbolic coup.

On a similar note, I think the March on the Morannon must have reminded Sauron of the Last Alliance of Elves and Men--not something he would want to remember. Admittedly, the number of Elves in this new alliance was only three if you count the Sons of Elrond, but the presence of the Sons of Elrond and the Northern Dúnedain in battle with his enemies of the South represented the alliance of their respective lands. This would serve as a reminder of Sauron's failure to divide his enemies against him, and would also serve as a reminder of past failures. After all, the last time that Gondor and Arnor fought together, the Witch-king's armies were destroyed, and the time before that was the Last Alliance.

Sauron, whose calculated cunning was very great, would have been extremely unlikely, in my opinion, to have ignored the symbolic victory the chance to wipe out these enemies afforded him.