[arp242]

Depends what you're writing.

If you're writing an installer script or whatever that's going to be run on $many computers that you don't control: sure, it's probably best to go to the extra effort to stick with POSIX sh.

But that's not most scripts. Most scripts are things that you run on computers you control. I just write things as zsh scripts because it's so much easier than bash (never mind POSIX sh). That's fine, because I can just install zsh. And actually, this often makes scripts more portable because you need to rely on external utilities a lot less.