I think it's a really nice idea :D Despite the fact that I was hoping that the 'Avaliable very soon' features would be implemented in the first place...
The Diary seems to be a great place to note down important, significant of just funny events from one's herd's life :o) Especially for players who take part in some Breeding Projects ^^ Writing down every success, goal, plan... sounds cool to me ^^ This is what I'm going to use the Diary for. I mean to make it my herd's Important Stuff Notebook, hehe xD
I just hope people won't use it as a chat, since that would make the 'Friends' Zone' messy pretty soon >.<