dash is not a bash "replacement". dash is an implementation of the Bourne/POSIX shell.
bash, when called as sh, also implements the Bourne shell as well (but badly, because it leaks bash-isms), but when bash is called as bash it is a super-set of Bourne.
It replaced what they were using previously, which was bash. I am not saying that it is a superset of bash's functionality or that it should be expected to support bash features.
EDIT: I see what you are saying now. Bash is still installed by default despite it not being aliased to /bin/sh. So it's still possible to rely on bash features if you use it explicitly. For some reason I was under the impression bash had also been aliased to dash in the default installation. Thanks for the information.
There was a lot of gnashing of teeth when we upgraded to Debian 6 and people's (alleged) "/bin/sh" scripts broke. Most folks elected to simply change things to "/bin/bash".
EDIT: I see what you are saying now. Bash is still installed by default despite it not being aliased to /bin/sh. So it's still possible to rely on bash features if you use it explicitly. For some reason I was under the impression bash had also been aliased to dash in the default installation. Thanks for the information.