[maccard]

Uf you don't fit into any of the categories of people that would normally measure their own Body fat percentages (athlete of some description, normally), your BMI is probably a good estimate of your "shape". Sure it's not perfect, but it's a perfectly good estimate for a large number of people.

[Symmetry]

I wouldn't call it perfectly good. It assumes that weight should increase as the square of height when really you should be using something like the 2.6th power. That means that taller people will have a systematically higher BMI than they ought to which can be misleading when populations get taller due to better nutrition, say.

[maccard]

Speaking as someone who is a tad over 6ft tall, and not a body builder, it fits me quite well. My bmi when I started paying attention was roughly 26, which I was shocked at, and I got defensive about very quickly saying "it's incorrect for tall people" but the fact of the matter was I was about 20 pounds (10kg) overweight. It also quite well describes my partner, who is about 5ft5-6