There is no reason why a 7th or 8th level Fighter would be a betterCharismatic Warlord than an Incubus Fighter 1.
Even with rules in place, there's still a story element to the game and a reason to choose certain enemies besides mechanical optimization. No one's going to look at a human warlord and say, "Why does he bother if a demon can do the job better?"
Sure the Pete with the Bard maxed out Bluff and castGlibness and has a Charisma of 22 and put a lot of mechanical
resources into being a great conman and liar character but Pete just cannot ROLEPLAY.
Then Steve playing the Fighter can roleplay but he has nothing to indicate that his Fighter is even merely mechanically competent as
a liar, much less "Potentially the best liar in the kingdom" that Pete's Bard character is.
Could Pete roleplay his Bard as having a Full Attack Bonus and Bravery and Armor Training and Weapon Training and Armor and Weapon Proficiencies? No. So why do you want the Fighter BBEG to "roleplay" being somebody with suave statecraft and master plans?
His role is to be the FIGHTER not that other stuff. Make the enemy a Magus or an Antipaladin or a Monster or a Bard or a Wizard if you want that stuff. "The Fighter" is limited as a BBEG because he has a limited role by his name...FIGHTING.
Well first off I feel like your analogy with Steve and Pete doesn't reflect the matter at hand because you're talking about two player characters. I said that the mechanics exist to facilitate the PCs interacting with the setting, be it through trade, seduction, murder, or whatever else they want to do. NPCs are a different matter; yes, they use the same systems when they're dealing with the PCs (or the PCs' NPC companions if they get involved), but everything "off screen" generally proceeds without needing to roll for anything. And if you don't need rolls, you don't need the numbers that exist to modify rolls.
Second, even if we are cleaving to the rules here, saying that the Fighter is capable of nothing beyond his namesake is flat-out ignoring the options presented. Granted, the class skills are limited and the skill points are few, but a good Intimidate or Profession (Soldier) is potentially enough to justify leadership depending on context. Plus, there is nothing at all preventing a Fighter from buying respectable mental attributes or putting ranks in non-class skills. And of course, archetypes like the Cad and Tactition open up new class skills and even more skill points in the latter example. So while the Bard might have an edge in those departments, there's no reason to flat-out say that Fighters simply cannot do more than bash people.
Actually, to follow up, "suave statecraft and master plans" aren't Bard class features either. They're encouraged to have high Charisma and have access to useful skills, but their innate performance abilities are generally more useful in combat than social maneuvering (except Suggestion, but using magic to influence people can easily backfire if anyone figures it out). Plus, there is no game mechanic for plotting and scheming; it can be assumed that high Intelligence helps, but nowhere does it say that there's any kind of minimum for having and executing good ideas.