While I agree Emma has thought meanly (contemptibly) of the Bates I do not think Mr. Knightley ever thought contemptibly of Harriet or that her good nature and sweet temper were contemptible qualities. Emma said “Harriet's claims to marry well are not so contemptible as you represent them” (8) which I interpret as he rates her claims to marry well lower than Emma’s rather than they are contemptible.

I also don’t feel that Mr. Knightley dismisses Harriet’s worth. Consider the context. He says her claims to marry well are no higher than Robert Martin. Granted a gentleman farmer is lower than Emma’s expectations of a gentleman in manner and education but no one else considers Harriet’s prospects so high. Thus rather than dismissing Harriet’s worth I think Mr. Knightley was saying what he felt inescapable about her claims to marry well.

I do agree Mr. Knightley did not do Harriet’s good nature justice until he was forced to learn it while they danced (38) but I still would not put him in the same category as Emma’s failure to admit Mr. Martin’s full worth. I think Mr. Knightley’s treatment of Harriet was always proper and gracious despite the fact he was not inclined to know her as more than a causal acquaintance and while Emma cannot be faulted for propriety of behavior she was never gracious to Mr. Martin until the last two chapters.

It appears Mr. Knightley asked to be introduced to Harriet before Ch. 5 or he would not have known enough of her character to say all he did to Mrs. Weston about Emma’s friendship with Harriet. After they danced Mr. Knightley revised his opinion of Harriet upwards but he also took an interest in her and subsequently found more to admire:

"And I am changed also; for I am now very willing to grant you all Harriet's good qualities. I have taken some pains for your sake, and for Robert Martin's sake, (whom I have always had reason to believe as much in love with her as ever,) to get acquainted with her. I have often talked to her a good deal. …I am convinced of her being an artless, amiable girl, with very good notions, very seriously good principles, and placing her happiness in the affections and utility of domestic life." (54)

Emma did not admit Mr. Martin’s superiority of situation until the second to last chapter:

“I think Harriet is doing extremely well. Her connexions may be worse than his. In respectability of character, there can be no doubt that they are…” (54)

Emma finally asks for his acquaintance and fully acknowledges his worth in the last chapter:

“Emma became acquainted with Robert Martin, who was now introduced at Hartfield, she fully acknowledged in him all the appearance of sense and worth which could bid fairest for her little friend.” (55)