Some regard Charisma as a 'dump stat'. It is a bit difficult to have a 'dump stat' when you are playing 3d6 in order, but you know what I mean; STR 18 gives you a +3 bonus in melee combat, DEX 18 gives you a +3 bonus to AC, CON 18 gives you an extra 3 HP per level. Even modifiers provided by high INT and WIS can provide mechanical in benefits; extra spells, higher chances of knowing a language, bonuses to Saving Throws or adventuring skills, depending on your particular flavour of The Game. But CHA? CHA? CHA 18 should give you a bonus to NPC reaction rolls and an increased maximum number of henchmen. But these are small beer; how many modestly endowed PCs are maxing out on their henchmen allocation? And how many Old School GMs set aside roleplaying to randomly determine NPC reactions? Or remember to?
Now me, well, I am comfortable enough with a Fellowship test, or the like, as in WFRP, to use some kind of CHA test to determine the success of a PC's attempt to inveigle their way into the Black Brotherhood, or persuade the Archgourmand of the Ogres that adventurers are poor, stringy, gristly meat. But even in those cases, it is the actions of the players, not the dice rolled for their PCs, that carry the most weight in determining the success or failure of their actions. The +3 to hit provided by STR 18 counts whether or not the player of the PC can actually lift a sword; the player simply says, 'I hit with my axe' and the GM says 'roll'. The effect of the +3 to NPC reaction rolls provided by CHA 18 is dependent on the ability of the players to put words in their PC's mouth.
Cha-, Cha-, Charisma eighteen, Russia's greatest love machine
Regardless, CHA can be made mechanically meaningful in an Old School game by making it every adventurer's Prime Requisite. And then some. A +1 bonus (CHA 13-15) = 10% extra XP, a +2 bonus (CHA 16-17) = 20% extra XP, and, naturally enough, a +3 bonus (CHA 18) = 30% extra XP. That is a lot of extra XP, but then someone with CHA 18 is a person that is more likely than most to turn the bare facts of their deeds into legend.
But then, for me, XP are not literally a measure of 'experience' - after all, what do PCs get XP for? Finding treasure, mostly. Sometimes killing stuff. And, if you are that way inclined, completing quest-like objectives. And accumulating this XP does what? Makes PCs better at fighting, others better at magic, and some better at sneaking about. The same experiences improve characters in different ways. Why? Because D&D is not a BRP game. D&D is a more abstract (and less 'realistic'). In my games, XP (and Levels) are measures of a PCs fame, glory, and legendary status - I've argued before, D&D [only?] makes sense when adventurers are 'rock stars' - though this 'legendary status' is only in part a social construction (boosted by carousing and conspicuous consumption too; you can buy 'charisma'), but is built into the physics of the game universe itself.
The idea of using the CHA modifier* to boost XP rewards comes from a number of sources: Mongoose RuneQuest II uses CHA to determine the number of improvement rolls that a PC gains per session/adventure, on the [dubious] basis that charisma helps PCs find tutors/training partners better than gold, or intelligence. Pendragon has a Glory statistic that is a combination of a Player Knight's repute and something more 'real'. Old School games have, since they were New School, awarded XP for carousing and the like (Chris Kutalik, of Hill Cantons, not only does this but increases PC Charisma scores as they level, looking at the relationship between CHA and level from the other end of the telescope [take one // take two // Hill Cantons Compendium]). But the tipping point came when I was reading some Pelinore stuff, and came across the Free/wo/men NPC class. "The level of a Freeman or Freewoman is not determined by experience points but by a combination of their wealth, age and influence". At first I though that NPC classes such as this are a great way of avoiding the situation in which every NPC of interest has an adventuring class and a few levels, or else is a fragile 0 level non-entity. But then I thought that the idea of levels relating to 'wealth, age and influence' might help us to think about what XP and levels are meant to/ought to represent.
*I am all about the modifiers these days. I'd rather have a PC roll 1d6/2d6/xdy (whatever) and add their modifier to beat a target number, than roll 1d20/3d6/xd6 (whatever) to roll under the relevant statistic. Using d6 rolls to resolve adventuring tasks obviously owes a lot to the streamlined D&D rules found in Lamentations of the Flame Princess. 2d6, though, gives a probability curve, and Stars Without Number (via Traveller) provides an example of how to use this in an Old School context. Dyson Logos has a nifty conversion of B/X Thief skills to d6 and 2d6.