"Who uses a 0.X version of a filesystem in production?"
ZFS as a filesystem has been out of beta for years already. And I say this because I've been working with ZFS on FreeBSD for at least 5 years now and before that have been using it on Solaris 10/x86 for another 4 to 5 years. And no data loss because of filesystem hiccups I might add. Heck, on FreeBSD I can even put my whole operating system (so: including the root / kernel) on ZFS which used to be impossible on Solaris (note: it's been a long time since I last used Solaris).
Now, I'm well aware that you probably meant to address the project and not so much the filesystem, but even so I think this could easily be a reason why some people may be tempted to use this. ZFS has been available and extremely stable for several years and better yet: it's been an open source project as well.
So I don't think it's that crazy that people expect at least some stability, especially considering the age of the filesystem itself.