You know, I know it was just an omake, I could actually see Shirou using Unlimited Bayes Works in a serious fic. Reality Marbles derive from minds which are alien to the common sense of humanity, and as we all know, humans are anything but properly rational. Kiritsugu Emiya already told Shirou the basics of his moral system in canon; it wouldn't take too much more elaboration for Shirou to pick up "Magi are supposed to be rational about doing good" as well as "Sometimes, in order to save people, people have to die."

Then he'd just throw himself into making himself rational the same way he threw himself at physical exercises following Kiritsugu's death (that is, with single-minded devotion to the practice), and before you know it, it'd be time for Unlimited Bayes Works. That said, if he did become a more rational person, he'd probably know that cleaning the Archery Club floors is not the most efficient use of his time, and tell Shinji to go fuck himself, thereby preventing him from seeing Lancer and Archer fighting, and from there Lancer's attempts at killing him and Saber's summoning, so the story would take a different turn, right from Day One.