] I wonder why JA bothered with Mrs Hurst? Perhaps we are to see her marriage as an example of what Caroline might end up with if she doesn't find a rich husband who has an estate?

I think her function plot-wise is to give Miss Bingley someone sympathetic to her opinions and attitudes. Of course, if Louisa were single there's a very good chance she'd be fighting with her sister for Darcy's attentions, and I think that's more than the poor man can bear! ;-)