why?
they are too, too thin an unwieldy. The flaps don't tuck in well, and if you tape/flag them down, the corners are all over the place. Never ship in them, because they usually split and burst, and the books fly all over.
I never save these when I receive books in them that have survived shipping - they go straight to the trash.
Liz, I love ya but that's not the best explanation. M2 stands for Mylites2, which is a 2 mil thickness Mylar and what 99.9% of folks refer to when they say "Mylites" or "Mylar."
The Mylites you are referring to are officially called "Mylites+" and are a flimsy, useless 1.42 mil thick.
IIRC, before "Mylites+" came to be, they were simply “Mylites” and were even more useless at a skimpy 1 mil thickness.
Link for noobs:
http://www.egerber.com/