"I think she is one of the most charming young ladies I ever met, and might have been most useful in such work as we have been doing."-- Sherlock Holmes, The Sign of Four
Sherlock makes that statement in his explanation of groaning at Watson's engagement announcement. He goes on to speak of Mary Morstan's genius, then suddenly veers into a statement that "love is an emotional thing and whatever is emotional is opposed to that true cold reason which I place above all things." He then quickly adds, "I should never marry myself, lest I bias my judgment."
Sherlock Holmes is obviously covering up something here. We know he's all about the work. Caught unsure of exactly what to say, he's going to default to talking about the detective business. And then talk about how emotions are something he doesn't do . . . right after he had a very emotional reaction. He had just audibly groaned "a most dismal groan."
Now, if you're a romantic, you can go two or three ways here. You can look at it that he himself was infatuated with Mary Morstan, John Watson, or both, and saw there marriage as an opportunity lost. That's a pretty easy route to take. And seemingly more probable than taking Holmes at his word.
Because if we take Holmes at his word, we have to figure out just how Sherlock Holmes was going to use Mary Morstan in their detective business, "such work as we have been doing," as Holmes puts it. But what work was that? He seems to be sitting around, bored and swapping out morphine and cocaine for three months when the case begins, even though he claims he's been consulted by a French detective, seemingly via the mail.
He's consulting by mail, Watson has written up their first case together, and the one thing he finds most impressive about Mary Morstan seems to be her ability to keep an important paper singled out from other papers. Was he hoping to hire Mary as a filing clerk for 221B Baker Street? And somehow Watson's proposal invalidated her from that job?
A more disturbing prospect for the role of this never-filled job comes when you consider that some Sherlockians place this case as occurring in September of 1888 . . . the month of the Jack the Ripper murders. Watson's sudden turn for making Mary Morstan a life partner certainly removed her from the possibility of Holmes using her for Ripper bait. Holmes's idea of the things that were a part of his work ranged very widely, so possibilities going from filing clerk to pretend prostitute are not so unimaginable.
But there is always the possibility that "work" was he and Watson's code name for something else, and the fact that Watson was going to make Miss Morstan his bride leaves Holmes with the immediate reaction of being "as limp as a rag for a week." I will, however, leave that one for keener minds on that sort of topic than my own.
The whole passage about the Watson-Morstan engagement and "such work as we have been doing" definitely needs more study though. And I can't help but wonder if Mary would be good for that work, were she around today.