Sorry if I was not clear in my statement. I like to view it as a jumping off point for character creation. Profession (class) seems a good way of determaning one's personality. Shy, social awkward people do not tend to make good salepeople; the same is true for bards.
Does this mean that you cannot have a bossy wizard? No...it just means he will ALWAYS be an intelligent wizard (if he's any good). I see no reason to think that intelligent != bossy, grouchy, charming, or any other adjective that doesn't imply stupid. Why could an intelligent wizard not be nobleborn or self-effacing or dashing? I had not thought I had implied such a thing.
A Warlord has shown that in some capacity, they must be able to lead through some of their abilities. I seem to recall one mentioned that, when a Warlord charged an opponent, any ally who charged that same oppionent got a bonus; another (Feather me yon oaf) in which the warlord directed the party to use missile weapons at one specific enemy on the Warlord's turn.
So even if the Warlord is a hired on mercenary and not Party Leader, his basic abilities require him to lead in some way (such as ordering a missile strike or leading a charge). It makes so sense for his abilities if he cannot lead. If every time he uses one of these abilities, the wizard roars out "I am party leader, no you! I make the decisions!" then the class becomes no fun to play because their is an implied need for the character to lead.
Just like it would be difficult to play a cowardly defender (possible, but lacking logical sence), I would imagine it will be difficult to play a leader class that has no believable ability to lead and inspire people.
Does being a leader make the PC party leader? No. But if you are making a character who picked being a Warlord to begin with, some aspect of leadership should be in his personality...why else would he be in that profession?
Can you have a Warlord who doesn't want to lead? Sure...the noble raised to rule, taught tactics and strategy; good student but one who never wanted the burden of leadership. But he would find that in using his class-given abilities, he was leading people. Sounds like the type of character that grows from the reluctant heir to the noble king (Aragorn much?).