Board Thread:Fun and Games/@comment-33328512-20171016194627/@comment-25376906-20171027193536

The Dunlendings were probably not interested in Mordor at all. It is Saruman that used them to wage war on Rohan and Saruman was trying to his own thing until he was ensnared by Sauron in the Palantir.

That aside, I don't think Dunlending warriors would be very efficient, especially compared to Dorwinion elf warriors. And they would not see any need in marching to Mordor especially through Rohan or Gondor.