There are several good reasons why we SHOULD see a Warlord class:
1: A significant number of D&D fans want to play a pure non-magical combat leader.
2: The Warlord allows the realisation of a common trope within fantasy novels, TV and film to be realised.
3: There are a wide range of interesting mechanical possibilities which the class could realise (many of which are likely too powerful for feats).
4: It would be a nod to the fans of 4th Edition that not everything which made up the D&D they love is being thrown under a bus.
5. It gives a non-magical alternative to spellcaster healing, allowing low- or no- magic campaigns to be run without extensive re-working or re-balancing.
6: Because the leader of men class has been part of D&D for over 10 years now. We've had warlord-like classes in 3rd edition with the Marshal, Warblade and Crusader.
And together those are MORE than reason enough for a class.
I'm still hopeful that we will see all the classes from every PHB. (azd the good that is served by delivering on that is likely a good 5th argument for the Warlord)
Heck I even think that they could do an interesting and engaging Assassin (using shadow) and Illusionist (using shapeshifting and mental effects) without treading on the toes of the Rogue or Wizard.
This will allow us to fully express the 4 key Warlord archetypes:
The warlord who is most concerned with maintaining and boosting his allies and keeping them in the fight. This warlord specializes in martial healing, granting temporary hit points, resistance to damage, and ending conditions. He does this through a combination of first aid, improvement to morale, and an intimate knowledge of how a body can adjust to and react to combat situations.
Signature Power: Martial Healing
A warlord who leads by example. He attacks enemies in ways that show allies how to fight them more effectively. This manifests as the warlord hitting a foe, thus granting allies a bonus to either hitting that same foe or hitting a similar foe (as in "Here's how you take down a giant!").
Signature Power: Impose a condition that encourages allies to attack that target.
A warlord who spots tactical weaknesses in position and directs allies on how to get the most advantageous position. This usually manifests as free movement, extra benefits from positioning, or off-turn parries.
Signature Power: Grant allies off-turn movement.
This is the warlord who oversees a battle and directs the allies to give them benefits to their attacks. Unlike the vanguard, who exploits an enemy's weakness, the captain deals primarily with his ally's own strengths, granting off-turn actions, added damage, or bonuses to attempts to trip, grapple, etc.
Signature Power: Grant allies off-turn attacks.
Rather than use his strategy to boost allies, he uses it to demoralize and disarray his enemies. With choreographed attacks that are designed to be as demoralizing as they are damaging, he causes enemies to miss opportunity (attacks), take penalties to hit or damage, or to incur fear-based conditions. Like the vanguard, his powers trigger off attacks, but unlike the vanguard, his attacks are meant to affect other enemies, rather than his allies.
Signature Power: Hit an enemy and impose a condition on nearby allies of that enemy.
Now, I personally think all of these would make fine styles for the Warlord. Heck, I could even see each of them as a separate "fighting style" as distinctive as the fighter's current styles -- two-weapon, two-handed, ranged, and sworn-n-board.
But I don't think it's realistic to expect the designers to make half of the fighter's fighting styles be warlord-themed.
And the 11 minimum Warlord features:
1: Grant advantage to the next attack made by an ally (and the Warlord also attacks).
2: Give an ally a bonus "free" attack with extra damage.
3: Grant bonuses (or advantage) to initiative checks.
4: Give an ally a "free" move (and the Warlord also attacks).
5: Give an ally a save against an ongoing effect (and the Warlord also attacks).
6: Use an interrupt to reduce damage to an ally.
7: Restore lost HP to an ally (and the Warlord also attacks).
8: Improve out-of-combat healing.
9: Allow and ally to move and attack off-turn.
10: Grant an ally a "free" attack with NO bonuses (and the Warlord also attacks).
11: Give an ally a bonus to damage on their next attack (and the Warlord also attacks).