I am not sure if he has been mentioned but I think Beorn is an excellent candidate for Sword-thain. 6 hit points make him ideal target for undefended damage then, and with some healing, you easily gain an action out of that. I even considered Defender of the West for being able to do that. Raiment of War makes things better still.

I thought about Beorn for that same reason, but then you'd never want to make use of his ability since you'd loose out on the attachments you have on him. And being a hero doesn't actually do anything for you. You mentioned undefended attacks, but Defender of the West already takes care of that - and can only go on an Ally anyways. So if you Sword-thain him after attaching that, none of the text of DofW applies anyways.

The attraction of ally Beorn is that his 3/3/6 combat stats are the best of any Tactics hero but hero Beorn, and this Beorn can take attachments. 3 attack is the highest non-Beorn value in tactics, and only Beregond has more than 2 defense.

However, all the other 3-attack heroes have a nice ability, and Beorn ally has an ability that's near-useless for a hero, and his inherent traits aren't much worse. ally-turned-hero-Beorn really needs something that gives value just from his good base stats, and there's not a lot. Hour of Wrath would work very well for him but is hideously expensive, Taste it Again! would be fantastic but requires lower engagement cost. Armored Destrier would be handy but he's neither sentinel nor leadership. Probably the best thing to do is to give him Raiment of War (4/4/8), Captain of Gondor (5/5/8) and then use generic readying effects to have him defend and attack. Raiment of War can be put on his ally form, of course, but there's a lot more readying options for heroes than allies.

A Christmas-tree Beorn would be pretty spectacular. Unexpected Courage, Rainment of War and then give him the Gondor trait with Steward of Gondor and put on Blood of Numenor and Gondorian Fire. Or just use Boromir and use the same attachments, skipping Sword-thain and the Unexpected Courage...

The nice point about Beorn is definitely his exceptional stats, so unlike an UC on Beregond - which is only useful for Defense - one on Beorn could be useful for defense or offense. Gives you a bit more flexability in the combat phase. And if you are desperate to finish something off, you could still use his ability, if it wins you the game.

I really like the idea of Sword Thane but the problem is that as you pointed out, without UC most of the allies just arent that impressive looking. Even with UC they're good, but spending a UC and a Sword Thane on them is quite a lot of investment (and you have to draw said Ally, Sword Thane, and UC before hand), it doesn't seem that practical to me.

The one exception I think is Legolas. Because Legolas has such a strong attack (and Ranged Keyword) and a strong ability, AND is in Tactics sphere (enabling Rohan Warhorse for readying instead of relying on UC), Legolas is probably the best and IMO -only- real target for Sword Thane shy of a Caldara deck where you are really just using ST to have another spirit hero.

Since it is mentioned so much everywhere, I think I haven't played Unexpected Courage since 2013, and I can't say I miss it, ever. Of course the card is good, but I just dislike the design because it is so wrong in comparison to many other similar readying tools, that I rather play without it. And a good deck should find ways to play without it. Elrond might still be one hero who needs it most, but I can just do with Miruvor instead, and in many instances, it proves better.

I had a deck that was about getting OHaUH Gandalf out and buffed up, and it originally had Sword-Thain, but in the end it was pointless to make him a hero. Some extra readying could have helped if it were a multiplayer deck, but in solo, it wasn't really needed. Narya also would have been worse with Gandalf being a hero. The only reason to make him a hero would have been to give him Song of Wisdom and A Burning Brand, which is just silly because that's 4 attachments that need to be played on a specific order: Narya, Sword-Thain, Song, Brand. Maybe it might be nice to get some other attachments him, but I'd have to give up something else to fit them in the deck.

http://ringsdb.com/decklist/view/598/gandalf-is-not-a-threat-3.0

Beorn with a Sword-thain could be a target for Support of the Eagles. Then, Unexpected Courage makes all kinds of sense. Also, maximum bonus for silliness.

Hero healing is a lot more common than character healing.

Yes, 6hp hero is good enough, especially in a deck with healing, you can let attack undefended on regular basis, which give you action advantage… the stats of ally Beorn are just amazing!