Intelligence is the prime requisite for magical types. Intelligence will also affect referees' decisions as to whether or not certain actions would be taken, and it allows additional languages to be spoken.
Wisdom is the prime requisite for Clerics. Wisdom rating will act much as does that for intelligence.
Charisma is a combination of appearance, personality, and so forth. It's primary function is to determine how many hirelings of unusual nature a character can attract.
Thanks for nothing, Gary. The Original Dungeons and Dragons attribute definitions are of little help in determining the appropriateness of Charisma as a prime attribute for Psionicists. In fact, the original definition of Charisma hurts the argument! It makes sense to use one, or a combination, of Intelligence, Wisdom and Charisma, but how to justify Charisma as THE attribute?
Turning to the AD&D Players Handbook, Gary offers a few more scraps from the table.
Charisma: Charisma is the measure of the character's combined physical attractiveness, pursuasiveness and personal magnetism. A generally non-beautiful character can have a very high charisma due to strong measures of the other two aspects of charisma.
Better. I like the pursuasiveness and personal magnetism angles, but can those be leveraged into a justification for Charisma as the psionic attribute?
Finally, I turn to the wikipedia, which knows all.
Charisma: from the Greek word "kharisma" meaning "divine gift".
Although difficult or even impossible to define accurately (due to an abundance of wildly diverse criteria in regard to the trait), charisma is often used to describe an elusive, even undefinable personality trait that often includes the seemingly 'supernatural' or uncanny ability to lead, charm, persuade, inspire, and/or influence people. It refers especially to a quality in certain people who easily draw the attention and admiration (or even hatred if the application of such charisma is perceived to be negative) of others due to a 'magnetic' quality of personality and/or appearance. Related terms and phrases include: grace, exuberance, equanimity, mystique, positive energy, joie de vivre, extreme charm, personal magnetism, personal appeal, "electricity," and allure, among many others. Usually many of these specific qualities must be present within a single individual for the person to be considered highly charismatic by the public and their peers.
Thank you wikipedia. Based on that definition of Charisma, I am far more confident using Charisma as the prime attribute for Psionicists.