Among other things, bash is useful when most of what you’re doing is communicating and piping between various programs (many of which are also standard). And there are many environments where bash (or at least a POSIX shell) is available but Perl and Python are not.
And of course, you should distinguish between interactive bash and scripted bash. Ubuntu recently switched their default scripted shell (#!/bin/sh) to dash because it was much faster. However, bash has useful interactive features dash does not (zsh is still better, IMHO).