Wizards want Intelligence and Clerics want Wisdom, because they're the only ones who can use those for magic.
No-one wants Charisma. Not that you'd turn down a good CHA score, but it's not core to being useful to any class.
So: Everybody gets the exact same benefits from those three physical stats, but the two mental stats have special effects specifically set aside for Clerics and Wizards. Training doesn't let fighter use his +3 Strength for any class-specific purpose, but only a Wizard's knowledge unlocks the potential hidden in the INT score. That's shitty for physical classes, and makes INT and WIS almost useless to everybody but Wizards and Clerics.
I think the best thing to do is to have six core classes. Each does something special with each ability score. We already have two flavours of spell caster - one for Int, one for Wis. It seems sensible to extend that and make a Charisma-flavoured Thief, and a Con-flavoured fighter. The Charisma thief would be some kind of Fool or Alice. Fighters could be split into offense and defense, barbarian and knight.
All the scores should have a massive effect on every character, regardless of class. This makes every character more distinct. If you can say "Hey, my fighter's pretty smart" - and that has a meaningful impact on the way you fight dudes - then you've got more options and a more interesting character. You already get bonuses like better saves and more languages, but I don't think that's enough - ideally every score should be as important as STR and CON are for non-fighters, and those are hard scores to top.
The spirit charms are my first step to doing that - everybody can use a Wis check to get help from the spirits, so everybody has a good use for Wisdom. I'm giving Charisma some heft with this Caste system. Charisma gives you your rank- fuck with anyone below you, fear anyone above you. For Intelligence, I'm thinking you can make an int check (d20 + int mod VS a difficulty) to know monster HP, AC, abilities, attacks, secret weak spots, etc - all but the most secret of knowledge.
I might work through this and post more later - rules for giving fighters and thieves something special to do with Strength, Constitution, Dexterity and Charisma, and those two extra classes.