The Elvenking stands as both a brilliant yet strange attachment.
As a 1 cost unique, but not restrictive readying attachment that must be attached to a Silvan Hero, The Elvenking has many pros going for it. An action to ready makes it a cheaper Leadership equivalent to the omnipowerful Unexpected Courage. A deck building joy for Silvan decks that don't use Spirit, as Silvans are a 4 sphere archetype, though the inclusion of Spirit should not discourage the use of this card. Additionally, an action to ready makes it more versatile to other cheap, repeatable readying effects, such as Heir of Mardil or Snowmane, which have conditions for their readying powers.
Arguably, the two main contenders for The Elvenking are Thranduil and Celeborn. Thranduil to allow him to defend twice with his 3 or Celeborn to allow him another action in addition to using his 3 (making Mirkwood Long-knife a great choice for the Lord of Lorien).
Whilst there are no conditions on the readying effect, The Elvenking does come with an additional "cost" to activate; to return a Silvan Ally to your hand. Silvan players will note the immediate benefit to this cost; presenting the opportunity to play the Ally again for it's entering play effect, which is the main gimmick of Silvan decks. A guaranteed source of Ally recursion in such decks is a major boon in many cases to generate additional value. It synergies extremely well with mainstay Silvan engine/force multipliers such as Celeborn and Galadriel, showcasing the narritive harmony between the two Silvan Elvish realms quite nicely.
However, this additional cost is not without it's drawbacks. One cannot activate The Elvenking if one does not have a Silvan Ally to return. This is admittedly, not the most pressing drawback. More pressing is that The Elvenking fuels the traditional central tension of Silvan decks; the cost/benefit analysis of reducing your board state to fuel cheap, yet powerful effects. Unless the flicker effect immediately returns the Ally to the field i.e Host of Galadhrim, you must sacrifice your board state to activate many Silvan cards such as Feigned Voices, The Tree People, Defender of the Naith and Pursuing the Enemy. Thus, to gain the benefit of such cards and have the returned Ally in play, you must play, and therefore pay for, the Ally again. This puts a heavy toll on your resources to keep your board state developed, particularly for tri or quad sphere decks (though Thranduil helps smooth resources). This tension does provide most of the interesting decisions when piloting Silvan decks though it can be mitigated through in sphere resource generating cards such as O Lórien! or Steward of Gondor. Admittedly, The Tree People puts an Ally into play, hence it's ubiquitousness in Silvan decks.
Furthermore, deck building potential is stifled by the parasitic nature of the additional cost of returning a Silvan Ally. For Trait specific cards such as Herugrim and Fair and Perilous, one can make creative deck building choices by including Trait bestowing cards such as Nor am I a Stranger or Elf-friend. The return of a Silvan Ally cost means the card is only useful in Silvan decks. Whilst one could include The Elvenking in decks that have some Silvan Allies, in my opinion, it would be too inconsistent to justify it's slots in a deck.
Overall, The Elvenking is a fantastic inclusion in many Silvan decks. It allows for Ally recursion to generate Silvan enter play value on top of readying a Hero. It facilitates many interesting decisions, just don't thoughtlessly destroy your board state. What can men do against such reckless recursion?