John had his medical practice and Sherlock Holmes's cases and his writing to occupy his time. Mary was used to earning her own way, and the boredom of housekeeping was driving her mad. Her solution was not ladylike but what of that? It was exciting and brought in a few coins.
Besides, hadn't Mr. Holmes himself had complimented her on her instinct for relevant details? And there was information a pretty young woman could get that even a great detective would be hard-pressed to gather.
John needn't think he was the only Watson to assist Mr. Holmes with his cases.