I was going to move on, but it's worthwhile examining these points from Ozymandias, particularly this last:
". . . you'd be taking something the players have right now (complete control over their NPCs in battle) and limiting it unless the fighter characters focus on leadership."
Unquestionably the answer to that last is yes, I would be adding some limitations to the existing system. I'm not sure what limitations yet, as I'm still formulating the specifics, but it stands to reason that any addition in rules will create limitations where none existed before. This is why 'rule' relates to a 'boundary.'
However, I wouldn't expect there to be much of an increased limitation. I've already built in two containment systems on the players' utilization of their hirelings/followers, the aforementioned veto and morale system. As regards most things in combat, my emphasis in rule-making would be to allow the character with leadership experience to circumvent that containment: to overrule my veto or to improve the hireling/follower's morale and make it less of an issue.
We have to understand, however, that for every hireling/follower that the party acquires, there is an endless parade of NPCs that the players do not control. Many of these have characteristics that simply prevent any possibility of alliance with the players. One does not simply convince Captain Ahab to change his mind and suspend his quest in order to ship the party to their desired destination. Othello is not going to become less jealous after sitting down with a player to have "a good talk." Becky Sharp will remain hell-bent on improving her social position. That's just how it is with these people; while not necessarily evil, they are obsessive and therefore strongly resistant to change - and so it goes with most of the population, to a lesser degree, as we all know from any personal exchange we've had with someone who has a personal political axe to grind.
D&D would have it that any good player ought to be able to radically change these circumstances with "really good role-playing" . . . which in turn puts the DM in the position of having to reward players who chatter well with an automatic NPC about-face. As someone who does argue very well, who goes at it like a pit-bull, I've seen this actually happen perhaps four or five times in my life - this coming from someone who has given it plenty of opportunity to happen. People are bloody-minded, stubborn, argumentative forces to be reckoned with, as the reader has already discovered having lived long enough to learn how to read.
It is possible, however, to play to what people believe in order to get them to do as we want, without needing to change their minds. It takes talent, however, to spy the belief system, recognize how to get the chisel underneath the person's defensiveness and distrust, then pry up the person's willingness to help out in just such a way that the person feels good about themselves when it's done. It is also a talent to recognize that person in a whole room of people who don't possess the sort of belief system we need for just this sort of operation.
This is what a leader does, however. A leader finds those people who are already predisposed to follow a certain banner, who then waves that banner in front of those people and gets them moving. Recruitment, therefore, is critical where it comes to building a team that can be briefed, right from the start.
Normally, if I had a player enter a town and try to hire anyone, they'd come up empty. The player characters are obviously strangers, they talk strange, they wear road-battered clothing, they carry weapons, they're mostly unwashed and they have no references of any kind. A person would have to be crazy to work for someone like this. Therefore, most of the time, the only hirelings that players have are a) associates that the background generator has given them; b) characters who they have aided or rescued in some way; and c) characters who happen to be going in the same direction or who have the same goals as the party. In the case of (b) and (c), it is the players' actions and decisions that makes the difference in those NPCs being willing to hire on and join . . . and I make the decision myself, based on whether or not the players have respected those NPCs.
The leadership skill would circumvent my decision. In effect, it would say that no matter what the players' actual words or personal treatment of the NPCs, the leadership skill would trump it. If the player, as my world goes right now, said to an NPC, "You idiot! Do that again and I'll kill you!", I would have the NPC fade. With the leadership skill however, I must presume that the character's words were much more appropriate, considerate and motivational. Therefore, the NPC would not fade.
In answer to Ozymandias, then, the ability wouldn't determine that the team would do something right or wrong based on the brief (technically, that would be an expeditionary/tactical consideration). Rather, it would ensure a greater chance of the Leader meeting and transforming strangers into allies, whom the player would then have the opportunity to run personally, with a greater morale in the face of danger, a greater chance of sacrificing themselves at the player's order (over my veto) and therefore offering a greater resource for the party.
Leadership doesn't make people better in terms of ability, only in terms of their willingness to act. Once having done so, we would rely upon the expeditionary/tactical ability of the player to pick the right people for the right job, ensuring probable success. This doesn't make individuals better - but it would allow individuals to work at their greatest efficiency.
I considered for a long time whether or not 'recruitment' ought to be an ability possessed by Leaders or by Trainers. I settled on Leaders; and as a proof, I offer the present military system in which those people who recruit new soldiers are completely divorced from those who train them. I see recognizing potential and enabling potential as both different process and mindsets.