>Bash is a lowest-common-denominator language, available on almost all platforms, with a very stable (old?) API, which makes it ideal for broad distribution & bootstrapping of systems.

/bin/sh is, bash isn't. Portability is a good excuse for using /bin/sh, bash is just a crappy middle ground, it's not as portable as the POSIX shell and it's still not a decent substitution for a proper scripting language.