Everyone plays an intricate role in the story......and since this is Kishi's story and if you are true Itachi fans you would understand that Itachi wasn't the type of individual to divulge that kind information....he was a mysterious character, and the fact that he did die revealed even more about the dark history of konoha and that the village isn't all its cracked up to be.....the fact that Itachi died made this character so much better....
Jiraiya is my favorite character and I know for a fact that if he would have entered that battle knowing about Pein, then he would've defeated him, but again his character that served a purpose to reveal some of the secrets of Pein....those two characters revealed more about Naruto and Sasuke's past that will ultimately put them on a path that they can never reverse......but that's just my opinion.