Only if baby Jesus is using an open source BSD. Bash is standard on all Linux and OS X installs, and vastly more featureful than the POSIX sh standard. If you're a decent bash hacker and really in a situation where you can't rely on a better scripting runtime to be installed, you'd have to think really hard about whether FreeBSD/OpenBSD is worth the hassle.
Honestly, the BSDs need to get their act together on this and either bash-ify their shells or just use it. Their existing environment is pretty poor.
(I guess an argument could be made that Android is another non-bash environment. But the shell there is so terrible that I literally don't know anyone that's used it for anything in production.)
(Edit: I guess technically I should include Solaris people in the list of bashless victims. But, well, yeah...)
It depends on what you're scripting, some things really are expressed best in the shell.
But tools like perl, python and ruby aren't nearly as universal as you'd like them to be. Obviously if you maintain the box and are deploying an app, they're trivially available. But if you're shipping software to be installed on end user boxes of unknown configuration, they're a dependency that can bite you. That's why things like self-extracting archives and shell-based initscripts persist, even though they look ugly to modern eyes. They do what they need to do very well, and they do it universally.
Depends on your target. Bash is the LSB shell. If your scripts are only targeting Linux, then you should be able to safely assume bash is there. On Linux, by default, there isn't even a real Bourne shell anyway; it's a symlink to bash.
If you're writing cross-platform then yes, sh would be a better target, but a lot of environments are going to be Linux only.
