I wouldn't call IQ a completely useless yardstick. Likely through examining all of the evidence regarding IQ and military performance, the United States government has come to the conclusion that an IQ below a certain level disqualifies the person for military service.

Unsurprisingly, US law is a bit complicated on this, but to summarize, it implies that an IQ below 81 (“tenth percentile”) is disqualifying, and dictates that persons with IQ's between 81 and 93 (“thirty-first percentile”) cannot comprise more than 20% of all enlistees.

It depends on how one defines intelligence and what specific things are considered material to the measurement. IQ tests measure the things that IQ tests measure. The challenge is that the things we often want to determine in trying to assess "intelligence" are multi-dimensional, multi-variate and have complex interactions.

Personally, I consider most typical assessments of a person's general "smartness" to approach zero utility.