Am not clear why some are so resistant about carrying one even if they're "certain" they won't need it (how can anybody be that sure?). After all, an un-used bag weighs almost nothing and takes no virtually no space.
The reason I was is because I didn't want to be wasteful by taking a bag then never using it. I suppose you can drop it back off some where for others to use but I never had that chance. But yeah, "you never know" has never been so true in these cases.
I do Whitney twice a year, and one of the items I pack is an unused wag bag from a previous trip. When I pick up my permit and handed a wag bag, I decline it, saying I already have one from a previous trip.
For me, this is one way of not wasting USFS resources.
