Here's my rationale for a detailed quest journal: I'm not my character.
I'd like to think I am, and I we sure act the same sometimes, but I'm a person living in another world, with things to remember, deadlines to meet, and names I constantly forget as is.
Our characters in Sui Generis would remember much more than we do as gamers. It's all they do. If they hear Jane Doe can only be saved from an ogre with an enchanted spade, our characters will remember something important like that. However, us as people have other things to worry about, and if we leave the game for a week and return, no time has elapsed for our characters (still rearin' to go and find that enchanted space), but it's not unlikely for us as people to forget the current objective.
The Journal isn't really for our character's purpose, but ours as extensions of that character. Adventurer Dan knows his current task backwards and forwards, so the journal is there to remind us of what we already know.