As the only 1-cost Eagle, this is highly desirable for expensive Eagle decks. It's not half bad outside of them either - as a matter of fact, the only other 1-cost ally in all of Tactics is Knights of the Swan, who's useless on his own. Vassal is a nice damage spike/potential chump blocker for little investment, in a sphere that tends to lack money. Also a good target for Support of the Eagles when you don't have time to build up an Eagles of the Misty Mountains.

If you dare try and make Horn of Gondor work in an eagles deck, this is the best chump blocker in the game. Essentially a free boost to Eagles of the Misty Mountains. The only problem? You have to have an Eagles of the Misty Mountains in play for this combo to work, at which point you may not be hurting for resources.

Not a ton to add that others haven't said, just one big thing. And I think most people know this by now, but these guys are like a cheat code for a scenario with Battle questing. 1 resource for 3 attack is amazing in and of itself, but the extra benefit is the fact that you ignore his forced effect since he's questing instead of attacking.

Same goes for Winged Guardian and siege questing. If you're struggling with Into Ithilien or Cair Andros, look no further.