Rob's aunt used to love getting garage sale sacks for me, which was very kind.
But if it was permeated with cigar or cigarette smoke, you know the kind, greasy yellow, into the burn barrel it went.
The same with musty/mouldy fabric. It is dangerous to be around, and not worth health to try to bring back to useability.
I haven't had any cross my cutting table for years, but I've never kept poly and poly cottons. I don't like the feel and once stained seemed impossible to clean when I used to wear it all the time.
Time and place for everything I guess. That said, I cut fabric down to 1.5" squares and have used thousands and thousands of them...bring 'em on
Good question Bearisgray, usable is different to all of us!