Initially I've been against it, but now I'm sold. Scribbling things on a piece of paper would probably be faster, but in game journal is much better roll-play wise. If it is implemented, it'd be really cool to see an option to copy/paste passages from books. Doing so probably require to drop some of diary auto fills and make reading books essential to completing at least some quests, therefore making the game to complicated for some people. So not gona be implemented. And that's something that's gona be hard to mod in because it requires rewriting the whole quest structure.
P.S. Sorry for crappy English. Not a native speaker.
It don't essentially want it as a quest recorder more of an optional diary for roleplayers like myself. Here is an example
'This man, Ulav, apparantly knows about my families death but according to other townsfolk he is a very powerful individual but also paranoid also he isn't going to invite me into his home.
...Not Yet'
Things like that, creating your characters backstory.