Remember, that the
POSIX shell and utilities are (almost?) always installed and readily available, whereas Perl and/or Python might not be.
In theory, you might be correct. There could be a machine which has only a Posix-compliant shell, and nothing else.
In practice, in the free software world (Linux, *BSD, ... even Raspberry Pi) that simply will not happen. Perl and Python are available for all of those; and probably even automatically pre-installed by the default install. The same is true on the Mac (which is after all nothing but a free installation of a Unix OS that comes with your hardware), perl and python are available and pre-installed. On the commercial Unixes (AIX, HP-UP, ...) the situation is a little more tricky, as the default distributions that come from the vendors (IBM, HP, ...) sometimes do not put Perl and Python on the machine, but these are always easily available as an add-on.
Matter-of-fact, you can more rely on Perl and Python than on bash. Personally, my habit (formed from having worked on projects that often used 3 or 5 Unix flavors simultaneously) is to rely on having perl or python, but then only use the minimal set of Posix standard shell, and not rely on bash.
Why do I say this here? Because learning shell scripting (meaning sh, not perl/python) is indeed a very good thing, as Sir Dice said. It's incredibly useful to do off-the-cuff three line scripts right at the command line, or put 20 or 50 line scripts in a little file and use them over and over. But one has to know the limitations of the technology. A 1000-line script in shell is a maintenance nightmare, and very hard to get correct and debug. At that point, one has to think about using appropriate tools, and instead write a 500-line program in perl or python that accomplishes the same thing, but can be written in a much more structured fashion.
So what I'm saying is: definitely learn how to do shell scripting. Whether you use a book, a web site, or just trial and error is up to you. But quickly afterwards, learn a scripting language that is more appropriate for larger projects.