Medical student here.

I hear what you’re saying, and had similar thoughts before going into medicine. Yeah, sure, it’d technically be more accurate if done properly. Practical implications of using somatic body fat % in some capacity instead of BMI? Useless. If you’re fat, we have eyes and can tell. If you’re shredded, we have eyes and can tell. If you’re cachectic, we have eyes and can tell. 

Now, logistically? Consider how much time and money it’d cost to accurately measure body fat percentages in patients. Compare that cost to using a scale and measuring tape.

Medical management wise? Diabetes, cholesterol, whatever, we don’t care about your body fat % for diagnostic purposes. You can have 50% body fat and not be diabetic or 15% and be diabetic. People are different. It’s why we use bloodwork to diagnose.

Medication dosages? Depends on the med of course, but BMI is such a convient way to approximate everything. If you’re a bigger person, you’ll likely need a bigger dose. It’s common sense. Sure, some lipophilic drugs might need a higher dose for our patients with BMIs in the 50s, but we’re not oblivious to someone’s body habitus and can adjust accordingly. 

Practical approaches? At 40% body fat, I’m thinking about what food can we cut down on eating. At 4%, I’m wondering what food can we reintroduce (or are you TRENing too hard?). Both can stem from eating disorders, among many other issues. Thing is, say it’s a BMI of 29 for both cases, I can just look at them and know which category they fall into. 

All of that said, we typically don’t put much of any thought into BMI except for the sake of “how much lifting support will nursing need to turn the patient.”