Question: Who decided that needed Unexpected Courage fused with Staff of Lebethron? Because that's what Armored Destrier is. Yes, it takes up a restricted slot, but shadow cancellation and readying in one card is still a really good combo. And, if you have Elfhelm in play, Armored Destrier also gives you a boost, making you miss the restricted slot it's taking up less. Gondorian Shield may be able to provide +2 , Blood of Númenor even more, but to me Armored Destrier is the king of the defensive attachments.
First off, who it can go on. I like how, while has the theme of blocking huge attacks (via Feint, Beregond with a Gondorian Shield, or Gondorian Spearman) has more of a theme of blocking lots of smaller attacks. Warden of Helm's Deep won't block as high of an enemy as Defender of Rammas, but he can block more attacks overall due to sentinel and the ability to survive direct damage. Armored Destrier continues this theme, providing no base boosts but readying the defender so you can deal with multiple attacks. In addition, it can go on sentinel heroes, allowing you to defend multiple times across the board.
Second, the readying. Unexpected Courage is almost a staple in decks, so readying is clearly a strong effect, and, while it is more situational readying than ye-olde UC, Armored Destrier does not disappoint. Elfhelm defending for 3 twice is nothing to sneeze at, or defending once so you can counter attack. Erkenbrand is another great target, allowing you to block twice with in-built shadow cancellation.
Third, Armored Destrier's shadow cancellation. If Armored Destrier just provided the readying, it would still be good, but the shadow cancellation makes it incredible. Discarding a shadow card is less accurate than cancelling one you know has a shadow effect, but it has its own benefits (die, stupid Core Set Wargs, die!) And, even if you do discard a shadow card with no shadow effect, that's still an attack you can feel safe knowing how much it's going to hit for. The shadow cancellation and readying on Armored Destrier make the Dream-chaser cycle a lot easier, allowing you to stay engaged with enemies like Throngs of the Unfaithful almost indefinitely at low to no risk.
Put all of this together, and you get a ridiculously good defensive attachment that can bypass two of the main weaknesses of defenders, multiple attacks and shadow effects. Personally, I'd even say Armored Destrier's a little too good, especially with Elfhelm in play. Seeing that you're engaged with 3 enemies, as a questing deck, and being confident that you can handle your defenses, is a little too much for me. A 'Limit 1 per hero' would be enough, though, at least I think.
One last thing: if there's one deck, more than any other, that benefited from Armored Destrier, I would have to say it would be Dunedain. is one of Dunedain's homes, making this in-sphere, and it fits their play style like a glove. Staying engaged with multiple enemies means you need to deal with multiple defenses, something Armored Destrier provides easily. Those multiple attacks leave you vulnerable to bad shadows, which Armored Destrier discards. This fits so well in a Dunedain deck (at least theoretically), it's almost ridiculous, though it is very Tolkien-y. An attachment for a new archetype providing a huge benefit to an older one, despite the fact that the older one is mentioned nowhere on the attachment. A chance-meeting, as they say in Middle-Earth, and one which brings great profit.