Protection from rain and TSA or equivalent dirty hands are definitely good reasons to pack in ziplocs. I once found "goo" on the outside of my checked suitcase when I picked it up off the bag belt. It was some kind of liquor that a dummie had packed in their suitcase and checked in. I managed to wipe it off with lots of wet paper towels, and the stuff didn't leak through the bag (thanks Skyway), but after that I started using ziplocs. I had to chuckle when contemplating what the inside of the dummie's bag looked like...