Haha.. great question Richie! I was wondering this just the other day and yes, I have received a couple of these too. I first suspected that the intention was to leave a backlink without having to put any thought or effort into it. After all, we are all writers, so how hard can it be to write something? Having said that - some of the people who do this are excellent writers who have left intelligent and thoughtful comments on my hubs. So perhaps there is another explanation?
I have sent "blank fan mail" myself, early on when I started HP. I think it's when you click an option like "good hubs" or "cool writer" and then send it. I thought writing something was optional when I started HP. Now i usually write something, but it's not clear what's required and the non-written option exists.