the most valuable multipurpose item in my panniers?
an 8x10 olive drab silnylon tarp with a lot of guyline. packs to the size of a small water bottle. Picnic table shelter, bike cover, tent vestibule, windscreen, privacy shade, camoflage, sorting sheet, clothesline, increased bike security at night.
if you get a 5x8 silponcho you add raincoat, but in most rainy conditions I prefer a cover over my seating /eating area versus wearing a poncho, but that's just me.
i think that's the one for me. Bandanas are right there too, i carry one everyday in my pocket, not just touring...