In deck "the-song-of-rebirth" (https://ringsdb.com/decklist/view/2452/the-song-of-rebirth-1.0 ),the builder use Warden of Annúminas just for its 2 attack. So it's hard to find ally with high attack. So Rider of Rohan is perfect for mono spirit deck:they can quest or attack and can do both with a small limit. spirit side quest is not bad,I'd like to add them to my deck.Totally worth it.
But if my deck don't have other attacker or defender, Rider of Rohan is more like a 3 cost 2 willpower, I'd like to use Galadriel's Handmaiden or some thing else rather than this ally.