The whole stats system of DnD (and similar games) is a mess because it's totally oversimplified and unrealistic, out of necessity and/or tradition. It's a big fault to use the same stats for combat and dialog for example. There are no reasons why a gifted sorcerer, a cleric, a fighter or barbarian cannot be charismatic, full of wisdom or strong and "intelligent" at the same time. The dialogs do not fit for a dumb person on the other hand, mostly. It's difficult to allocate most of the options to a person of low intelligence and at the same time stay consistent in behavior.
I think there is no way to make this system better without huge effort, so it's best to send your common sense on vacation and just overlook the many faults of the system. From this point of view it would be best to have freedom and variety for stats disposition for as many races as possible, which speaks for the introduction of the variant human.
