I see this as so wrong.
In cRPGs, the PC is the leader.
In PnP, the 'leader' can be anyone the group chooses to follow.
Again, I think you're focusing too much on stats. There's no such thing as a 'gimp character that only comes out during fights.' The character is always there. Maybe half her face was mauled bear, she bathes once a month, and she only speaks in monosylables and snarls (which would give you far less than 8 charisma), but she most assuredly has a presence and can influence the entire party.
Leadership is based on personalities within the group and the situation. The idea that can guess before hand who will be leader based on one stat is, I think, ridiculous.
That's the definition of charisma. You may argue that there a broader measure is required / optimal, but technically a character with low charisma cannot lead a group ... perhaps she might be able to threaten, bribe or extort a group to do as she wishes, but they will only voluntarily follow based on their reaction to her charisma.
Yes, players' innate characteristics (and their supernatural knowledge of their PnP characters' world) do affect the interactions ...