Rider of the Mark, at first glance, seems like a reliable way to waste spirit resources. He is however not a bad quester, has an extra hit point, and has built in shadow cancellation. If you don't quest with him and use his ability, he can swing for one to help kill that shadow-less enemy. One thing to note is that since the shadow card is discarded, you can get around effects like Smaug's and those pesky Wargs' that reference different things on the shadow card itself.
Rider of the Mark
Action: Spend 1 resource to give control of Rider of the Mark to another player. (Limit once per round.)
Response: After Rider of the Mark changes control, discard a shadow card dealt to an enemy you are engaged with.
Road to Rivendell #33. Spirit.
All around solid card. I found his ability to be especially useful when paired with Gimli because it allows you the luxury of taking an attack undefended without fear of any nasty surprises and placing the damage on the son of Gloin. Our dwarf can then swing back with a boosted attack. Typically you have to block with Gimli first before he gets boosted, but with this card you don’t have to which gives you a tempo boost over the encounter deck.