Yes the diary is a must. Its not a real hassle, just put all your scrap working out, designs etc in it. Everything that is not in the folio they look for the in the diary. Marks will be given for stuff in the diary.
Its helpful because it shows your design from conception till the finished product. Thats what the markers like to see, your MDP as a "Journey"
I'd strongly advise to keep a diary.