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The very first reason is "1. The person who dreamed up the BMI said explicitly that it could not and should not be used to indicate the level of fatness in an individual.", which is the exact point being made: at a population level BMI is useful.

Additionally, BMI is imprecise, but imprecise doesn't mean useless. If you took a random sample of people with a BMI of 40+ from across the whole population, almost none of them would be near optimal healthy weight; a random sample of 30+, still relatively few. Those few who are healthy at that BMI know it. BMI's maybe not so useful for someone who's hovering around 19 or 28, but at the extreme edges it's very indicative of body fat.