Well no one would be anything without their knowledge, so I don't think it's fair to compare Fred's knowledge in physics to Buffy's superpowers, to me intelligence and knowledge isn't really a power. Remove all my education in chemistry, and my knowledge of horses, and all I'm qualified to do is work at the Double Meat Palace. Actually this is what we see with Buffy, due to her responsibility as the slayer, she was not able to obtain an education in anything other than the forces of evil, which is why she works at the fast food restaurant.