Honestly, with our densities, that hierarchy does feel redundant. Few realms have enough nobles to justify two armies. The days of having 15 titles spread over 45 nobles are gone for most people, many realms having more titles than nobles and most having a ratio near 1:1 (one way or another; all titles considered). Though I seem to recall that when marshals were created, realms often just had only 1 army anyways, or 1 normal and 1 elite, or 1 normal and 1 bureaucrat, etc. Whatever the number of armies, there was usually a single main one that did almost all of the fighting. Blob tactics and all command.
