Rohirrim Captain was never played. No, really, I never saw anybody playing it.
TMOR simply runs it over any day, unless u are doing some crazy deck I can't even imagine.
I would only compare Eomer 3rd Marshal to
Horsemaster and to
Heir to Meduseld:
3rd Marshal: almost always a base strength 9, but needs a horse... with a couple spears and javelins, could be consistently 11... important, tho, is he's usually 7 against dunland and a few orcs, and almost never bigger than 9 against orcs again.
Heir to Meduseld: potentially 14 base strength, at least 10 if you keep the rule of 6. This guy is awesome, tho most people don't really play him, guess he's just not popular.
Horsemaster: almost always 9, potential to become 11 easily, 15 if you really need it, but tricky thing is Rohan is always filled with possessions, and you'll hardly have many Rohan cards in hand by the skirmish phase.
That's just my 2 cents, being an Eomer fan
