Maybe we should look at this from another angle. What is the advantage of having a high Charisma in the real world? The advantage is that you are better at interacting with other people. This includes other people being more helpful, particularly in the sense that you have an easier time getting them to do what you want. You are also likely to be better at inspiring others to perform their best. Furthermore, high Charisma may also make you better at understanding others, that is, give some insights in guessing their motives.
This leads to a central point: If Charisma is to be reasonably well balanced, it needs to be thought into the quest and world design itself, and not so much into game rules. Many of these things are difficult to do without changing significant portions of the game. We could remove perks like Black Widow, Lady Killer and Child at Heart, and let the corresponding dialog be dependent on Charisma. Some quests could also be dependent on the PCs Charisma. For instance, Lucy West may only approach people that seem trustworthy (decent Charisma), or Vance may only teach a certain perk if the player has an insight based on Charisma.
In Fallout 2, your followers could perform some skills for you. Getting a few extra followers were often worth it as they could help you with your weak points in terms of actually being able to use skills aside from shooting up your enemies. Again, this would require significant work, as I'm sure Tarrant and others can elaborate upon. A somewhat easier approach could be to let Charisma improve the performance of your followers. As an example, each point of Charisma could add +10 HP and +1 to each tag skill for each follower, as a measure of them trying harder.
I think one of the issues is that this game doesn't really take advantage of dissposition as much as oblivion did. Say what you will about the oblivion speech challenge mini-game, but it did mean that high personality characters would have an easier time, in conjunction with speechcraft, to raise NPC's dispositions. This would also help reduce the price of goods.
From my point of view, the speechcraft minigame in Oblivion were one of the things that made Personality useless. Even with low speechcraft and Personality you could get the disposition of NPCs high enough to only need slight bribes (or charms) to get what you needed from them. I would prefer a more skillbased system, where disposition (based on past actions and Personality) would set a speechcraft difficulty. If you speechcraft were at the required level (or higher), you would succeed. If not, you would either be unable to get what you want, pay the NPC a large bribe or something similar.