Wandering Ent is one of the worst ent allies in game. Not because it is a bad card by itself, but because other ent allies can do anything he does better. Depending on what you want to use Wandering Ent for there are always better alternatives:

     If you want to defend : Use Derndingle Warrior warrior instead. At the same cost he has the same by default and can provide up to 5 when using his Action.

     If you want to attack : Use Skinbark or Quickbeam instead. Skinbark costs one ressource more but provides double the attack points of Wandering Ent which is         worth in my opinion. Quickbeam obviously is strictly better at attacking than Wandering Ent because of his readying response.

     If you want to quest : Use Quickbeam or Wellinghall Preserver instead. Again, Wellinghall Presever is more expensive but also provides a lot more value with one        more and the additional healing effects.

Unless you try to flood the table with as many ents as possible there are better options than Wandering Ent.

He might be the vanilla ent, but that is exactly is strenght. Yes, skinbark is a better attacker and derndingle warrior defends better, but what if what you need in that exact moment is willpower and not defence. Whellinghall preserver his better defending and questing, but he costs 1 resource more. Quickbeam is just bonkers. —
Because he can be useful depending of what you need at the moment with a good cost-stats ratio. Although he isn´t the best because the game rewards specialized allies, he is by no means useless. —

This card is viable in any deck that has a consistent defending hero. However I like this card especially in combination with Heir of Mardil. With both attachements attached to a high hero you can defend twice and get a resource for free. Obviously the question arises on which hero to play this combo. Without any further attachements only Aragorn, Théoden, Théoden and Gimli match the requirements. However I use it most often on Elrond where I use Dúnedain Signal or Elven Mail to give him Sentinel.

I very much agree with your above assessment. I'd add that you could make even more use out of it by using on Erkenbrand with Armored Destrier. —
I don't think this card comboes with Armored Destrier since you have to exhaust to add 1 resource so even if the defending hero defends multiple attacks it matters little. —
I think that Grimbeorn is one of the best heroes to get this onto, so that it can help paying for its ability. —

I have been using this card with Théodred. it is amazing. you can use it on theodred of you can use it on another noble character such as Faramir, Aragorn, and Thorin Oakenshield so you can get good use out of there stats

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If you're playing Guarded Attachments and Leadership anyway, then this card not only lets you find the specific Guarded Attachment for your situation, but it's also a deck thinner. Since the Guarded Attachments cost 1 anyway, it's a net cost of 0 (though it does mean you'll be using Leadership Resources instead of whatever you would if you drew and played the Guarded Attachment normally, but that may actually be a good thing, and even allows for playing out of sphere Guarded Attachments). Meanwhile each copy of this, like Daeron's runes, is effectively reducing your minimum deck size. Additionally, as the comment above notes, you can control the timing of the attachment (and thus extra enemy/location) coming into play.

The downside is that there are times you just don't want your Guarded Attachment to pull another Enemy/Location into staging, and this card will just sit in your hand, but if that's a frequent worry then you possibly shouldn't have those Attachments in the deck anyway, with or without this card.