Beorn is the BOMB. I don't care what you say about his art, 3, 3, and 6 is to good to pass up. Sure, he's expensive, but with that ability, who cares? Sneak attack, Imrahil, and Hirgon all just give the old guy new life.

Horseback archer is solidly ho hum. He's a bit expensive, but ranged is always nice. Mostly superceded by marksman of lorien, trollshaw scout, and Galadhon archer, he hasn't seen much recent play. But if you want a ranged attacker in a Rohan deck, he's pretty good.

Gondorian spearman is a very good chump blocker. He's fairly cheap, and can attack a bit before you need him for defense, or quest with visionary leadership. With Thalin and a spear of the citadel, he can take out 160 of the game's 531 enemies. Get a few of these out, play a stand together and hold the line, and watch as the enemies go down.

Kind of a bizarre card, in that it's nearly identical to Born Aloft and even came in the same cycle. The only distinctions are that this is an event rather than an attachment, so you don't have to anticipate needing it in the planning phase, and it only applies to eagles rather than all allies. In practice, you can almost always predict which ally is going to want the effect, and eagles are a very small subset of allies, making Meneldor's Flight straight-up inferior to Born Aloft.

In any case, common uses:

Spamming Descendant of Thorondor
Getting cards onto Eagles of the Misty Mountains in a pinch
Immediately usable if you draw it after the planning phase, or play eagles outside of the planning phase
That's about it... It's a really lame card.

Not a card you would think of as being very notable, but ends up being useful in a lot of decks. You never actually build around the mount trait unless using Elfhelm, but often have a couple in your deck that play a key role - think Armored Destrier + Beregond, Asfaloth + Glorfindel etc. Because they're usually unique and/or restricted, and many of them are meant for specific heroes, running three-ofs isn't very palatable either. Westfold Horse-Breeder is a good, noncommittal way to boost your chances of finding that mount, assuming you have access to the blue moneydollars. It also acts as a deck thinner if it hits. A one-for-one willpower ally/chump isn't bad anyway, so it's not a big deal if you miss. Also benefits from some powerful Rohan effects (Théoden, Ride to Ruin, Helm! Helm!...)

In short, consider using if you have a hero and a high-impact mount.