To all of you who responded that you don't use "thin" fabrics, I have to ask if you use batiks? While I wouldn't define them as thin as in "scanty" - they ARE thinner than a lot of other cottons - for example a Hoffman Asian. The difference is notable yet they are both good quality cottons - neither being "scanty".
I have used Batiks with heavier cottons (like Asians), and I won't do it any more. My main reason is that I like working in small pieces, and when you work with small pieces that teeny tiny bit of difference in the seam allowance will matter! If you use the same seam allowance (when sewing) to sew a Batik to a Batik, a Batik to an Asian, and an Asian to a Asian - you will have three different "finished" seam allowances after you press the seams. When you are working with a finished 1/4" piece ... you have trouble with a capital T. I for one do NOT want to adjust my needle position for each combination of "weight" that I use.
Just tonight I finished putting a border on a WIP. The border uses 6 different fabrics randomly in a "mosiac" of 5 1/4" squares. The next time I attempt a border like this, I'm going to make sure that all my fabrics are the same "weight" or "thickness" as some were not the same - again - these are all name brand quality cottons from LQS.
So even though you are buying good quality cottons, there are still differences.
Even within one manufacturer, I can find different weights in their fabrics.