This is great. First the bit about "the world's only anything", which is just a perfect little touch, and then

He chose to observe rather than inquire

Such a well-observed point. It's precisely why Holmes comes to want the doctor at his side. Watson is a reliable companion who won't bother him with needless questions. He'll take answers when Holmes is willing to give them, and is content to wait in the meantime. That's a very rare trait, and Holmes is particularly lucky to have a companion like John Watson.