Is BMI still relevant?

Introduction
BMI, short for body mass index, is a measure of the ratio between the height and weight of a person. It is used as a medical definition for the weight class of the individual. Several categories of BMI exist; these categories vary depending on the institution and geographical location. Typical categories include:
 * Underweight: Less than 18.5
 * Normal weight: Between 18.5 and 25.0
 * Ideal weight: Between 20.0 and 22.0
 * Overweight: Between 25.0 and 30.0
 * Obese: Over 30.0

Arguments for
BMI works well as a measurement for average people who do not have an excessive amount of muscle. It is a useful heuristic for quick assessment of one's weight-related level of health. Though "better" measurements exist, such as body-fat percentage, BMI should be sufficient for most people.

Arguments against
BMI only takes into account height and weight. It does not accommodate for physical fitness. A sufficiently muscular individual will find very little practical use for BMI, as it may estimate their weight level as "overweight" or "obese" despite being in good physical condition conversely (and much more often) it may say a sufficiently under-muscled individual may be classed as normal despite being obese when measured by more specific methods.

Discussion
Whilst outliers, such as the bodies of extreme athletes, may have physiques which are not accurately represented by the BMI level the vast majority of the population should use this as a base metric for health. This is confirmed by the studies mentioned above (reference) which have correlated the BMI classes above normal with increased health problems.

Similar arguments which suggest those who are taller and smaller are not accurately represented can be referred to the new BMI calculation which has weighted the calculation to take this into account. The influence this has on the BMI level is rarely more than 1 or 2 points, however, thus this factor also has little significant influence on the outcome.

There is also a correction that can be applied to account for differences in trunk length vs leg length.

Some arguments against BMI seem to attack a straw man, suggesting that the measurement is intended to be an all-encompassing measure of one's health. No proponents of BMI are actually arguing this. BMI measurement is intended as a heuristic; it is naturally inferior to a full examination from a qualified physician.

Alternatives to BMI
As discussed above, the amount BMI can determine about health is limited due to the inaccurate quantification of components such as muscle. An alternative metric, in terms of health, is Waist to height ratio...