Comparison of fiber:
All the photos below are Merino Wool at different weights:
Knit Picks Lace Weight
Shadow Tonal Lace Yarn
440 yards/50 gram hank
Knit Picks Worsted
Swish Worsted Yarn
110 yards/50 gram ball
Handspun Thick and Thin Yarn
Yellow Tan and Pink - Lemonade Stand
Approximately 63 yards/ Weight: 68 grams
So, if I buy 50 grams of Merino Wool, it will work up the same size regardless of weight. Yardage is not helpful to me; weight is way more important. It is important to understand that different fibers will be heavier and others are lighter... So weight in that case is a variant; but again if I am comparing the same kind of fibers; the yardage doesn't matter.
So, why do we state how many yards? I think because patterns state "so many yards" and it has become a standard. And yards are an easily measured thing. But what about how many yards of yarn does it take to make an adult hat? Well that is not a simple answer. I can make an adult hat with 60 yards, 90 yards, 120 yards, 200 yards, and if I want to go down to lace weight or thread even more yards would be required.
I am not sure if I have muddied the water, or helped clear up the mud a little. But it is important to understand that yards are not always the important information when buying yarn.
What are your thoughts?