I just went through mine and can see why: I have dozens of messages from non-friends on Facebook, and 100% of them were spam. If Facebook made this non-friend inbox more visible, they'd need to improve their spam-filtering.
I just went through mine and found quite a few relevant messages that I completely missed, which I would have liked to have received before the events themselves or before deadlines.
I contacted a journalist through her Facebook account and today just heard from her thanks to the Slate article. She was sorry she did not see my important email.