Yeah, all I can think of is that at least the point of it in the ep seemed to be at least in part to signal Something Different. Not a new thing that was going to be a staple in the show from now on--at least I hope not--but just to say that something new was going on with the character. And that something new had to do with being introspective and looking at his actions, so they probably thought a diary was the most obvious thing.

And of course there were other things going on in the ep where there was no voiceover so those played more like the show usually did. Also now that I think about it, I think they saved the voiceover for things that were pretty obvious perhaps to signal that these were things that the character had a handle on. So it wasn't like watching him work through his feelings on something where we'd be interested in where it was going if that makes sense.