Think someone on reddit posted that the other day, I wanna try it stillI ride with a backpack but it's hella uncomfortable after a while, especially if you haul a lot of crap around like I do. But if you can, strapping a backpack to your rear fender/passenger seat isn't a bad idea.
Ive seen riders carrying a spare helmet in them ...thats gonna hurtId have thought it would offer a little more support when falling.
Guess it depends on whats in it