In your analogy that calculator would only produce a correct answer 80% of the time, and plausible looking but incorrect ones the other 20%.
If that were the case I’d hire pen guy.
More like “Producing 80% of the correct answer” and the remaining 20% with some nudging and tweaking. Still extremely valuable.
What's the error rate of the pen guy?
Also, if your AI has a 20% error rate, you're not holding it right. You need to spend more time keeping it on rails - unit tests, integration tests, e2e tests, local dev + browser use, preview deployments, staging environments, phased rollouts, AI PR reviews, rolling releases. The error rate will be much closer to 0%.