actually, i think it might be more about generating revenue for facebook. I've received very little spam in my 'other' box, and a spam filter would probably be pretty effective at reducing that anyway (were it even an issue). I think the spam thing is just an excuse, so facebook can give you the option to pay $1 to get your message to the recipient's regular inbox.

But yeah, this is old news and I'm surprised anyone is just now learning about it.