Having just taken Theoden, Thalin and Boromir through the Redhorn Gate I can say that he is definitely underrated. If not for his +1 boost to willpower both of my other heroes would have died at the final quest phase.
But outside of "low willpower means you can't X" effects for tactics decks what else is he good for?
Well for starters his boost applies to himself, so right out of the gate he is questing for 3. You add in his family Eowyn and that jumps up to an 8. He also can make Thalin contribute to the quest instead of just by damaging enemies. Characters with 2 willpower like Mablung and Merry also pair well to potentially put your starting questing power at 11 (when combined with our aforementioned Rohan heroes).
But the hero I absolutely love to combine him with the most is Boromir. His natural readying effect is great but adding an extra willpower didn't seem worth the threat before. Now that it's doubled you can quest hard with all of your heroes, and still have super Boromir standing by to ready and deal with any enemies that pop up. It's a great way to quest powerfully and race through scenarios.
Here is a link to the above named deck:
Sword-Thain also has added value on your unique tactics allies to add another hero. Golden Shield also combos well with its king. And if you add some Songs of Travel into your deck you could potentially get Herugrim, Snowmane or Steed of the Mark as well.
In closing for sure there are better heroes. But I would argue that there has not been a better hero for Mono-Tactic solo decks. As he corrects the main weakness of Tactics since the beginning. And appropriately I believe he was primarily made for the solo mono-tactics deck which fits the theme of the cycle he was release in. If you haven't given the Rohan king a try, he is that missing piece for you mono-tactics decks.