In practical terms I need a typechecker to write code at anything like the rate I do, with anything like the defect rate I achieve. But that's possibly sophistry. I agree the principle is not quite right (or at least not quite absolute) as stated; using extra power has a high but finite cost, it is occasionally worth using more power than you strictly need if the benefit is high enough.
But anyway, that's kind of my point. Typechecker is the kind of excess feature the principle of least power would like you to reject.