I remember reading someone else bring forth an interesting theory that Mary is part of the progression of Mr. Bennet's disinterestedness in his marriage and the upbringing of his children.
To paraphrase at much as I can remember (and I may look at references in the book that might reinforce the theory, but don't have time to examine at the moment...),
When Mr. Bennet first got married, he was in love with his beautiful wife, not realizing how much of a silly woman he had married. Jane is a happy child, because the beginning of the marriage was about mutual happiness between father and mother and family.
Then it becomes apparent to Mr. Bennet how silly a woman Mrs. Bennet really is, Mr. Bennet becomes a bit disillusioned, and pokes fun at his wife's foibles, still taking an interest in the family and raising of his daughters. Lizzy is the product of a father who still paying attention to his daughters and their upbringing, but is highly aware of the defects of his wife.
By the time Mary is born, Mr. Bennet has started to withdraw his attention to upbringing of the new additions to his family. Mary is the product of a child aspiring to be sensible like her father, observing him, but not receiving any of his attention.
Kitty and Lydia, Mr. Bennet pays no attention to, having resigned himself from trying to improve the mind of his wife or of the rest of his offspring, although Jane and Lizzy have already benefited from his early attention and guidance and there is continued interchange due to their intelligence and knowing how to seek or draw him out. So the youngest are left wholly to his wife and her "instruction" and guidance. Mr. Bennet stays in his library.
It's the not the perfect theory, or I haven't articulately well enough, but I do find it interesting, and I think I see Mary's place in the family as a bridge between Jane and Lizzy, and then Kitty and Lydia.