Yardage or Total Weight of Yarn??? I am confused!

I am asked all the time, "How many yards is in that skein?"  The question baffles me.  I can spin up 1 oz of fiber and get all kinds of different yardage depending on the wpi (wraps per inch).   And the wpi determines what size hook/needles I need.  However, one ounce of fiber is one ounce of fiber.

Comparison of fiber:

All the photos below are Merino Wool at different weights:

 Knit Picks Lace Weight
http://www.knitpicks.com/yarns/Shadow_Tonal_Lace_Yarn__D5420166.html

Shadow Tonal Lace Yarn

440  yards/50 gram hank

 Knit Picks Worsted

http://www.knitpicks.com/yarns/Swish_Worsted_Yarn__D5420153.html

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

http://www.etsy.com/listing/106209692/handspun-thick-and-thin-yarn-yellow-tan

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?