But why would you switch back to bash? Even if you don't use some of the advanced zsh features?
No, it isn't. Here's the Unix spec (http://www.opengroup.org/onlinepubs/007908799/), search it for bash and you won't get anything. That's a Linuxism (http://www.google.com/#q=linuxism), but it's not even on every kind of Linux.
The only standard shell on Unix is the bourne shell, and having things break because someone ignorantly hardcoded bash without using any bash-specific extensions is really annoying.
I agree it's worth the time to find a shell you like and customize it, but that's not a good reason to pick bash.
I think people are better off coding to a standard shell (as necessary), and otherwise using whatever shell makes them happy for day-to-day use. bash isn't actually the former, though, and it has strong competition for the latter, so it's probably a bad compromise.
I've also done porting for OpenBSD, and the whole bash thing is a pet peeve of mine. It's not just me, though - look at the situation with Debian and eglibc due to bugs like these (http://sources.redhat.com/bugzilla/show_bug.cgi?id=3266 http://sources.redhat.com/bugzilla/show_bug.cgi?id=9901).
* I mean, how many other problems in software are due to people prematurely overgeneralizing?
Sure. I'm just saying that, in my own case, switching from zsh to bash has meant that instead of worrying about making sure that zsh is installed and up to date on all my systems and that all the various addons and plugins I used to need for zsh are available, I just have to make sure my .bashrc is available. And even if it isn't I don't lose much. So I get a functional, familiar environment in all the cases that matter to me with no extra hassles.
BusyBox combines a bunch of Linux userland utilities into one executable, to save overhead on embedded systems. It uses ash, not bash.