Two things for me are:

  - Do I trust my code to run on a machine where /bin/bash doesn't work?

  - Do I trust my users to have their PATH configured correctly?

IME a user misconfiguring ~/.bashrc is about seven million times more likely than some theoretical argument about "portability", or even the idea that running my code on some unspecified version of bash that a mac user accidentally downloaded while screwing up a Homebrew copy/paste command is preferable to using the factory default that everyone has.