I guess you have to look at it from the social perspective of those times. When you look at the societal levels, Elinor is telling the truth that Mrs. Jennings will not improve their consequence.

Yes, they did mostly go out with Mrs. Jennings, and the only reason they went to the party with Lady Middleton, was because Mrs. Jennings was taking care of Charlotte.

The Miss Dashwoods had no greater reason to be dissatisfied with Mrs. Jennings's style of living and set of acquaintance than with her behaviour to themselves, which was invariably kind. Everything in her household arrangements was conducted on the most liberal plan, and excepting a few old city friends, whom, to Lady Middleton's regret, she had never dropped, she visited no one, to whom an introduction could at all discompose the feelings of her young companions. Pleased to find herself more comfortably situated in that particular than she had expected,

So it appears Elinor's worries were not founded. It's so funny that Lady Middleton regrets that Mrs Jennings has not dropped these unworthy city friends. Look at the Steele girls, whom Mrs. Jennings brought to Barton, but it is Lady Middleton who is so pleased with their flattery. There's some irony for you.

It goes back to the outward respectability of John Dashwood described in ch. 1. John and Fanny Dashwood, or Sir John and Lady Middleton would have given Elinor and Marianne more consequence in society had they brought them to London. But who would want it? I can't imagine Elinor preferring them. Well Sir John is OK, but Lady Middleton might cancel out the goodness of Sir John.

There's this tug of war of sorts in all of Austen's novels really between societal expectations and people with good sense. She points out the ridiculousness of all sorts of people. I'm thinking a bit of Persuasion here, where Anne finds herself at home with the more humble folk than her own set.

I know I would much prefer good-hearted people no matter where they came from.
I hope these ramblings made some sense and actually applied to the topic. :-)