I was talking to a friend about potential upgrades to his Joe deck when, with a flash of insight, I remembered that this card exists. Aside from the obvious point that Scientific Theory boosts the two stats Joe uses to win the game, there are a few reasons it sits well in a Joe deck.
Composure cards can be protected from horror by assigning that horror to other assets with sanity values, like allies. Joe Diamond, with his low and unremarkable sanity, already benefits from multiple high-sanity companions. Charisma works well, as does a Miskatonic Army style of deck, which runs Calling in Favors and series of cheap allies with abilities that trigger after entering play -- all such allies have two sanity. They may not allow Scientific Theory to say in play forever, but they will let it last longer than it otherwise would.
It is worth noting that The Circle Undone gave us Mr. "Rook", a very powerful, disposable ally -- he only needs to stay around for three turns. Mr "Rook" improves any Miskatonic Army he enlists in. Though he lacks the Miskatonic trait making it more of a Mr "Tonic" Army.
Mr Diamond can pony up the resources to get use out of Scientific Theory. His cycle also gave us Crack the Case, which is one of the best level 0 economy cards, and he frequently spends an action playing an event for free. If you go without taboos you have access to Milan money (ideally with Charisma), and if you go with them Scientific Theory shoots up in value because its biggest competitor, Higher Education, is a harder sell.