How is that portable, across different architectures?
I have to work in Solaris, SUSE SLES, some other linux where I don't know what they run, and OS X and I don't have root on most of those. bash seems to work the same everywhere and always seems to be there when I get access to a newly provisioned environment.
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.
I have to work in Solaris, SUSE SLES, some other linux where I don't know what they run, and OS X and I don't have root on most of those. bash seems to work the same everywhere and always seems to be there when I get access to a newly provisioned environment.