In a Fellowship with lots of Gondor and Rohan heroes it can be a good mount to add (and fetch with the Westfold Horse-breeder). Its effect is costly but in some turns can be really worthwile and it's not unique (like Light of Valinor and Snowmane), not one-shot (like Cram and Lembas) and less conditional than some other readying effects.
Steed of the Mark
Attachment. Cost: 1.
Attach to a Gondor or Rohan hero.
Response: after attached hero commits to a quest, spend 1 resource from attached hero's resource pool to ready attached hero.
...fresh horses were always in readiness to bear his errand-riders to Rohan in the Noth... The Return of the King
The Morgul Vale #139. Spirit.