Great question, Reeba! It seems like every time I even start to feel bad for John, he goes and does something to make me want to box his ears, largely for the reasons Robbin points out.

John does sometimes have genuinely positive sentiments towards his sisters, in contrast to Fanny's. I was intrigued to find that, when Fanny first rejects his idea of inviting Elinor and Marianne to stay with him, claiming that they should not take the girls away from Lady Middleton, John, "but with great humility, did not see the force of her objection," and even directly opposes her: "They had already spent a week in this manner in Conduit Street, and Lady Middleton could not be displeased at their giving the same number of days to such near relations." But his stand has little staying power, and does not survive Fanny's resort to the feeble, and instantly contrived, invitation to the Steele girls. Even before he mentioned the possible invitation to his wife, he looked to the expense and inconvenience of the step to assess its appeal. And Fanny does turn him round rather easily.

I can't blame Fanny for all John's disinclinations to goodness--once she has convinced him to wait a year before inviting his sisters to stay, he privately warms further to the idea without further input from Fanny, "slyly suspecting that another year would make the invitation needless, by bringing Elinor to town as Colonel Brandon's wife, and Marianne as their visitor" (emphasis in original). Indeed, he is far too ready to get off the hook by inappropriately passing the buck: "He had just compunction enough for having done nothing for his sisters himself, to be exceedingly anxious that everybody else should do a great deal; and an offer from Colonel Brandon, or a legacy from Mrs. Jennings, was the easiest means of atoning for his own neglect."

So, while I cannot feel entirely sorry for John as is, I do feel sad for the loss of the "amiable" John Dashwood that might have been, if, as Robbin notes, he had married someone who would have lifted him up rather than grind him farther into the mud.