In my experience, there's a huge amount of work involved in just getting Ansible (and other CM systems) to work right in the first place, especially if you're relatively new to CM.

Sometimes you're working on a scrappy prototype and it's just quicker to use shell scripts initially and then go back to solidify the setup into CM scripts afterwards, once you know what you actually need.

Of course that approach will fall to hell if one never gets back around to making the CM scripts.