How many non-server machines do you find yourself regularly needing a shell on where downloading it isn't an option? Why does it need to be pre-installed?
On the other hand reliance on bash-specific features is a net negative for GNU/linux.
The most used shell in *nix land is bash, but bash is significantly bigger than POSIX. This in practice means that many shell scripts are more bash-compliant than POSIX compliant.
It is part of the reason why it was good news when ubuntu/debian decided to transition to dash.
