Yes most of the time the respecting POSIX to the letter is not needed, but it is of course satisfying knowing that your script can run fine on BSDs and other less common distributions :)

Though sometimes you don't need to go that far to break stuff, for instance switching from Fedora to Ubuntu. I've seen many scripts fail on debian derivatives because people think using #!/bin/sh as a shebang is fine since it works on their computer where sh was in fact a symlink to bash.

But on debian based distributions /bin/sh is often dash, not bash, and dash is basically the strict POSIX subset + local, all fancy stuff like [[ ]], &>, arrays, ... will fail.

Though this is less about long options here and more about general shell scripting.