I disagree. George Washington or Alexander the Great were both charismatic and brave, just to name a few.
Remember also that "charismatic" is not necessarelly the same than "eloquent". Batman speaks very little.
Just because they are some examples of charismatic and brave, doesn't mean there aren't as many examples of charismatic and craven.
Just change "charisma" for "personality". Does it work?
Not really, no. Personality is subjective. Charisma seems to be the "measurement", and I use that loosely, of how effective a character can be, stat-wise, in social situations. Eloquent or lack thereof is more a factor of how the player views the character.