> There was once a poll of engineers at the tech company I work at, and I think the results were like 70% INTJ, 25% INTP, and 5% other. If MBTI lacked all predictive power, you would expect a distribution roughly equivalent to that of the general population.

You're assuming the types are proportionally represented in the general population, which we have no reason the believe even if the test works as advertised. For all we know urban environments are 70% INTJ.