"That is no orc horn..." Nope. It's tons of cards entering your hand every game.
This is the card draw Rohan has been thirsting for. Until Horn of the Mark was released there were limited options for ensuring those allies and tools would see your hand in time. Horn of the Mark is THE card which almost single-handedly rounds out the Rohan archetype as the final packs of the game are released. It seemed odd to me that Horn of Gondor previously had one of the best synergies for "ally leaving play" attachments in Rohan decks. HotM replaces this without question, especially with Théoden helping reduce ally costs already.
While Spirit/Rohan card draw may be a blessing in itself, Horn of the Mark can pair with hobbit decks to some extent so long as Merry joins the party. Who knows, perhaps Nor am I a Stranger may even make it's way to more decks. With the primary action ability of Rohan discarding or leaving play the payoff is huge, and even more so when Gamling is there to save them. Even the cards which temporarily put Rohirrim onto the battle field such as Lothíriel and Horns! Horns! Horns! will get more cards into your hands for no extra cost.
Horn of the Mark has quickly become a Rohan staple. If you love building Rohan it is a must.