From what I can tell Saladin is taking a page from his own personal life as a writer and is incorporating it into Miles' personality going forward. I remember in interviews, Saladin mentioned he didn't really see any major Marvel characters who were "writers", other than supporting casts who were journalist. It's kind of similar to how ITSV turned Miles into a young street artist, only this time with words. As for the journal narration I think it suits its purpose for now; but eventually, especially after it falls into the wrong hands, Miles will wise up and not write down his Spidey adventures in it, and Saladin will most likely switch to a new form of narration, while keeping the writer's spark alive in Miles in some other way.