Good questions. I can't speak for Martin, but I think he attached activity to the recent activity box so that students can see that stuff is happening in the class. It makes a huge difference to a student to see that others are working (and perhaps that he is not).
I'd recommend keeping it on. A room without action gets pretty quiet. There is nothing like a flurry of activity from kids to inspire others.
I would avoid journals entitled: Enter evidence here to tattle on your friends... If that journal had a lot of action, the student might become quickly unpopular.
