I've done quite a bit of experimenting to find (for my use) the lightest weight fabric that will get the job done. I started with heavy weight coated cordura nylon, then went to 1.9 ounce uncoated nylon, then 1.5 ounce uncoated nylon, then 1.1 ounce uncoated nylon, then to about .7 ounce uncoated nylon.
I figured I would eventually have a catastrophic failure and then go back up a couple notches for fabric choice. To my surprise none of the fabrics failed. The worst has been pin holes on the bottom or sometimes small tears from hooking on some sharp object. Nothing has happened that would affect the function of the pack bags on a trip, however (no big tears, no ripping at the attachment points, etc.).
All of the fabrics have been exposed to at least 30 hours of actual use with at least 30 lbs of weight. None have been exposed to abrasive rocks or serious thorns.
So I'm curious. What has been your experience? What do you consider a minimum weight of fabric for your pack bags?