[ Sorry your comment too so long to appear. The moderation queue for the blog has been inundated with spam, so I didn't see your post. I tried Akismet today and managed to clear the spam and dig the real comments out.]

To the actual topic – I’m not sure it’s more “extraordinary” to have all alot of universes or to have just one arbitrary universe.

Here’s another way of looking at it: Imagine we had a permanent cloud cover and couldn’t see other stars. Would a claim that there must be many stars out there be ordinary? I think it would, if plausible evolutionary scenarios of the universe are taken into account. Otherwise, we would have to assume that our solar system configuration came into being in an arbitrary fashion.

Similarly, it would seem that our universe came into being in an arbitrary way if we claim it is the only one. That seems extraordinary.

Another way of looking at it is by Occam’s Razor. The explanation we should accept is the simplest that fits the facts. A universe has quite a few arbitrarily chosen variable values. It would be quite a long description (“Our universe is such that X, Y, Z… and it is the only one that exists”). In contrast, “All Universes Exist” is a very compact explanation (possibly also the most compact explanation possible).