Couldn't the same be said for a "fighter" or "barbarian" or "thief"?
Sure. You could do away with the class system entirely.
But D&D is based around certain classic archetypes that are represented as classes. First the fighter, magic user, and thief/rogue, then the cleric, then ranger, druid, bard, and so on, then barbarian, monk, assassin, and so on. The warlord (or marshal, or leader, conceptually) is either another level down that list or not on it at all.
Like I said, the concept of a "warlord" or a leader who enhances other characters is compatible with most of the classic archetypes. It's very easy for me to imagine a fighter or barbarian or knight/paladin who is charismatic and considers himself a warlord. Cleric warlord? Sure. Wizard warlord? Sure. Rogue warlord? A little iffy, but doable. Conversely, a wizard thief doesnt seem like it should work without multiclassing; the same for a cleric wizard. Basically it boils down to what has historically been in D&D, but also how the game is played. What 4e calls a leader has always been more a theme than one class and has never been the sole imperative of any class (excepting the marshal, a bit of a strange one).
So I don't think it would be relegating the idea to make it a theme, I think it would make a lot of people happy (although not everyone, I'm sure).