Yeah, I remember that theory. The one where it suggested Smoker and Tashigi would both join. It was a solid case, but I do feel like Tashigi, and Smoker, will ultimately see themselves as marines before Strawhats.
I support the notion that pirates and marines are two sides of the same coin in a gray world, and to highlight that, there needs to be equal quantities of certain pirates and marines. In other words, there are good and evil pirates, as well as good and evil marines. Smoker and Tashigi are among the good marines, which remind the audience that the marines aren't all bad. That sort of gets lost if they become Strawhats. And given the story is largely told from the pirates perspective, it's more imperative these characters are associated as marines, otherwise it further tips the balance of perception in the pirates favor.