My main objection to it is that since the OP is almost certainly going to accept their own answer before other people can post, there may be other, better answers that arrive late and don't get as much attention.
Also, the SO/SE people have long maintained that Google is their homepage, and from this view it's easy to see why they would encourage people to answer their own questions: so that SO gets the googlejuice instead of some blog.
On the other hand, to the degree that we believe that individual blogs are good, we absolutely should worry about auto-answering.
That's an interesting concept that I'd never even considered. I then realized that I couldn't even remember what the stackoverflow homepage looked like, yet I utilize that website daily.
It doesn't seem to be harming the stackoverflow and associated sites, so I guess it's not a bad thing.