Not every Unix-like OS comes with bash, and certainly not by default. In particular, Solaris ships with a particularly crippled /bin/sh, although they do have a proper POSIX-compatible Bourne shell at /usr/xpg4/bin/sh.

If your Solaris machines have /bin/bash available, consider yourself very lucky.

dash is portable in the sense that Debian needs it to be: they can port it to the architectures and systems that they need it to work on.

dash is also portable in the sense that you need it to be: it implements the POSIX shell spec and not much more, so it's likely that you have access to all its features when you need to use another shell as well.