The category captain has been one of the pillar of the category management concept: The retail picks one supplier, based on its expertise, its vision of the category, its market share to work hand in hand in the definition of the key component of the category: The concept, the merchandizing, the product range...
Here is an excellent article that is questionning this category captain concept. The key issue highlighted is the fact that category management is not dealing with customers at all. It is a partner between to business entities trying to boost their respective sales.
Indeed, let's not forget that in category management, a category captain, as good as it may be, is still interested in selling its products.Hence, he may not be as objective as it may appear.
Category management must still be mastered by retailers, even though obviously retailers may be a great source of knowledge. But part of my experience, it is true that the category captain concept has gone in limbo.