I know I've read a fic where Watson essentially whores himself to Holmes to keep Holmes from falling into those black moods that make go for the cocaine.

However, it was one of those fics. You know the type. The type where everything is framed in the context of "d'awww, look how important they are to each other!" and the author is either too young, too oblivious, or too deluded to even consider the situation to be dubcon or emotionally abusive, let alone warn for it.