An attitude "It's just a shell script" can be dangerous when you have software that is supposed to be built on multiple platforms. You can never be 100 % sure what your /bin/sh is, be it bash, ash, dash, ksh, zsh or even an ancient Bourne shell.
Thinking like that leads down the same rabbit hole where autoconf/automake disappeared, only using bourne and m4 (!). Just in case you want to compile your code on a decade-old copy of HP-UX. Or I suppose some shiny new distro that is using zsh as /bin/sh.
It doesn't take care of the problem, it mitigates it. It tries to find the most POSIX-like shell. You still don't know which shell that will be. And then, even if you did, you are still relying on external programs, which may all work differently than the ones on your box. Writing portable shell scripts is hard.
