I was reading the passage describing Mary's time in Lyme:
"Mary had had her evils; but upon the whole, as was evident by her staying so long, she had found more to enjoy than to suffer." Etc., etc. to ..."and all this, joined to the sense of being so very useful, had made really an agreeable fortnight." (C 14)
I think this is an example of Jane Austen's irony that is so delicious. She is not berating Mary in the prose, but underneath is the meaning that Mary was functionally on holiday/vacation rather than being 'of use' to Louisa or the Harvilles.
How ironic that the useful individual (Anne) was not allowed to stay in favor of the hysteric sister who "had the best right to stay in Henrietta's stead!". (C 12) Yet the best way for Mary Musgrove to be useful was to take herself away from the invalid and be out of the way of those who were giving actual care. She fulfilled that usefulness very well indeed. I doubt anyone complained about her many outings. ;)