At 50-65% accuracy rates it’s pretty useless even for binary decision making (better/worse).

I care if my scale at home is accurate. I don't want to know if my weight is going up or down, I want to know what I currently weigh so I can decide what I need to do about it.

Your example seems to disagree with your premise. If someone tells me I weigh "9239" this means nothing to me, unless in context of whether it's going up or down, and how it is related to others of my height (and age and gender, perhaps).

I don't care that I weigh some multiple of an SI unit, or some fraction of a car. I care about my weight relative to others and my history.

No, it does not. Say I'm 6' tall. "Normal" BMI is between 18.5 and 25, which translates to between 136.4 lbs - 184.3 lbs. Telling me my weight is going up or down tells me nothing about whether I fall into this healthy range. And, depending where I fall (below, within, or above the range), the actions I would wish to take differ. It's that simple.

What instrument do you have that measures your "weight relative to others and your history"? Don't you only want to measure it relative to other healthy individuals? And, if you are particularly underweight or overweight, wouldn't you want to know when you're in the normal range? An instrument that only tells you what direction the change is doesn't tell you any of that.

But weight by itself (and BMI) are fairly crude measures. You could be 6' tall, be a muscular 210 lbs with a body fat of 10% and be classed as obese. The next level of the "onion" is body composition (fancy scales do this, but not that well). Then you're getting dexa scans....and there are probably other levels after that. I think I'd rather know if my weight and body fat % are going up or down, rather than their absolute value.

Le sigh. I should have known someone would throw that particular nitpick.

That’s an edge case. People who fall in that category already know that. And they’re very interested in their accurate, absolute weight.

Besides, BMI is just a convenient way of quantifying what a “healthy weight range” is. I could have used any range and said I was interested in weighing between X and Y.

I am kind of in that category...and I'm interested in relative fluctuations in body fat % even though I don't give much credence to the absolute accuracy of the body fat measure.

If you care about your weight relative to others, then it has to be accurate, no? Because how would you compare your weight unless you had a bunch of people using your scale.

True, but given that it's more difficult to change your weight than it is to find someone of a different weight, I'd rather live my whole life with a scale that might be reading (say) 5lbs high or low, rather than a scale that fluctuates between 5lbs high or low from measurement to measurement.

Sure. Weight relative to 1 lb or 1kg is essentially the definition of “absolute weight.”

Cant you just use the mirror for that?

Hard to use a mirror in your sleep.

Your mirror’s hard? Let me take a rain check on that, mine’s sleeping.

Uhh what? They were talking about weight

It's also difficult to weigh yourself with a mirror.

How would you even know if it is consistently inaccurate? The variability in biological signals might be so high day to day that it could be highly correlated but with frequent pathological cases that make the data useless.