But it doesn't seem like "fairness" is the issue to me. The DM is basing his decision on roleplaying aspects. He doesn't think it makes sense for the wizard PC to have leadership based on the character concept the player went with. To me, it seems like the player is using "roleplaying" and "character concept" as an excuse to get to powergame.
My concern is that the GM is essentially telling the player what his character concept is. The GM is not basing his decision on roleplaying aspects, he is dictating roleplaying aspects to the player based on his own preconceptions. It is fine for the GM to make rulings based on in-game situations, but in this case, the very thing under discussion is the in-game situation. The GM says this PC would never attract followers, the player says he would like to. The rules say that if you want followers, if you take the Leadership feat, you succeed.
The GM does not really have the prerogative to say,
"This character is not that combat-oriented, you cannot take Weapon Focus."
"This character is not that diplomatic, you cannot take Skill Focus (Diplomacy)."
"This character prefers wands to staves, he would never spend that much on a staff."
"This characters is not that sneaky, he can't take a level of Rogue."
What, exactly, is the GM's basis for saying this character concept does not include followers? If there is a Cha minimum, does that imply dwarves as a race do not have as many leaders? If the GM does not like the idea of this strange wizard having a cohort, he is basically saying his concept of the character trumps the player's.
If the GM is literally claiming that Leadership is unrealistic without a high Cha, then A) he is wrong, as outlined above, and B) the leadership chart needs to be rewritten so that Cha 15 or whatever now nets you the lowest number of cohorts and followers rather than a fairly high number. Clearly, if Cha 11 is insufficient to have followers at all, Cha 12 should not get you a small army.