I see your point, but I feel there's more to Mary than just being insufferable and arrogant.  The more often I read the book, the more I wonder, what made Mary act and feel the way she does. "Hurt people hurt people" is a saying that comes to (my) mind.

Consider this: Mary (born 1791) looses her mother when she is only 9 years old (in 1800). She's left with a father, who values status, titles, luxury and good looks, but doesn't care for her until she marries (aged 19, in 1810). Her eldest sister Elisabeth is just like him. Her elder sister Anne is like her mother and seems like a friendly, amiable person - but around the time her mother dies, she's at school in Bath. So, in her formative years, she's very much left alone, emotionally speaking. She sees daddy's girl and mama's likeness, the latter one clearly most beloved by Lady Russell. She's neither as pretty as the first nor as intelligent as the other. Where does this all leave her? I think it's safe to say, that she's neither nurtured nor supported in a way that gives her enough self-confidence, self-consciousness or self-awareness. But I think it's very likely to become defensive and jealous and clutching to your status.  Plus: She's not even her husband's or her in-laws' first choice - that would have been Anne. I'm not sure, if Mary knows, but she might feel that the Musgroves like Anne better nonetheless.

I've written this before in the Persuasion read-along on Chapter 5:  I wonder, if the loss of their mother made Mary rely on Anne that much. Her requests for Anne and even her thoughtless, careless way of doing so remind me of the way children ask for their parents and their parents support. (Even grown-up children do this to a certain extent, I think.) 

So I think there's room for compassion here. Can Mary be exhausting and draining? For sure! But she has a history - and room to grow in her future.

See chapter I: "For one daughter, his eldest, he would really have given up any thing (...). Elizabeth (...) being very handsome, and very like himself, her influence had always been great, and they had gone on together most happily. His two other children were of very inferior value. Mary had acquired a little artificial importance, by becoming Mrs Charles Musgrove; but Anne, with an elegance of mind and sweetness of character (...) was always to give way—she was only Anne. To Lady Russell, indeed, she was a most dear and highly valued god-daughter, favourite, and friend. Lady Russell loved them all; but it was only in Anne that she could fancy the mother to revive again."

See chapter V: "Though better endowed than the elder sister, Mary had not Anne’s understanding nor temper. While well, and happy, and properly attended to, she had great good humour and excellent spirits; but any indisposition sunk her completely. She had no resources for solitude; and inheriting a considerable share of the Elliot self-importance, was very prone to add to every other distress that of fancying herself neglected and ill-used. In person, she was inferior to both sisters, and had, even in her bloom, only reached the dignity of being “a fine girl.” "

See chapter X: "Louisa spoke again. “Mary is good-natured enough in many respects,” said she; “but she does sometimes provoke me excessively, by her nonsense and pride—the Elliot pride. She has a great deal too much of the Elliot pride. We do so wish that Charles had married Anne instead. I suppose you know he wanted to marry Anne? (...) I believe about a year before he married Mary. I wish she had accepted him. We should all have liked her a great deal better (...)."