[andrewmcwatters]

You should care about POSIX Shell Command Language compatibility because targeting anything above that will eventually require you to do more, not less, work.

If you haven't run into this, you just haven't written enough for the shell yet.

Once you run into it, you'll not want to write anything specifically for Bash or Zsh anymore unless you know beyond a shadow of a doubt that it will never be run in any other shell.

[dolni]

Nearly all of the shell scripts ever written don't need to run on Linux, and BSD, and Mac, and/or whatever esoteric Unixes that are still out there.

If you are writing a shell script with the intention of running in many places, fine.

But if you do that, the POSIX compatibility will be the least of your worries.