I like to think "mark as best answer" dispenses a little elf from AB towers who scurries to your house and pops a tube of smarties through your letterbox.
Well exactly- mark best answer does appear to do nothing. I think it's quite nice to be able to see which answers have been appreciated by other readers, and how many times. Otherwise answering questions can be kind of a lonely business.
I'm not sure I see much of a point to it and I probably wouldn't use it (except to be sarcastic), but then I don't like/use it much on facebook either. I don't see the benefit it gives to the site.
In the end we'll have a "like/mark best answer" button which does both mark as best answer and publishes it to your FB wall. Depending on whether you're logged into FB and whether you've given us permission of course.
Well I've just clicked on 'Mark Best Answer' and it appears to do nothing that I can see. YouTube also has a system where you can click thumbs up or down.