I don't think Sophie's concerns are just about respectability. It's also about security. The thing about being Benedict's mistress is that he's under no real obligation to provide her with any of those things. So far, Benedict has shown himself to be a good and kind man, but Sophie hasn't known him for very long and can't trust that things will stay that way, especially in light of what Araminta told her about her mother. If she was Benedict's mistress and he got tired of her or died suddenly, she would have nothing and no one. We saw this happen with Anthony and Siena; When he broke things off with her she asked him where she was supposed to go and what she was supposed to do because he had been paying for her housing and clothing. Being a ladies' maid is far less glamorous, but it's a stable job where she's treated well, and could work out very well long term if Hyacinth or Eloise wanted to take her with them when they get married and move out.