So coming back from Tesco's t'other day with a back pack full of eggs, washing up liquid, mince, tins of tomatos etc was a "bad thing". Better not mention the stuff I had stuffed down the front of my jacket then, let alone the fact that the loaf of bread I couldn't fit anywhere else was wedged in the fairing, obscuring all the clocks, idiot lights etc...
I wear a backpack quite a lot, there's sod all room under the seat, there's no rear seat to bungee stuff on, and I'm not gonna put a rack on it either. Ok, so I might be worse off in the event of an off, but seeing as the 'tona is my only means of transport, I haven't got a lot of options really short of selling it and getting a nice beemer or summat with hard luggage or more of an 'all rounder'. Some bikes just aren't practical for carrying stuff on.
I guess we're really talking about 'damage limitation'. Where do you want to draw the line? Ultimately riding bikes at all has it dangers, so personally it doesn't bother me any wearing a backpack, it's simply necessary for me and accepted as par for the course with choosing to ride what I do.
Footnote: Sh!t happens, an acquaintence of mine wrapped up a van against a tree a few years ago, he wasn't wearing a seatbelt and was lucky enough to get thrown through the windscreen. If he had been wearing a seatbelt (as common sense and the law dictates), he'd be dead now, the front of the van was completely stoved in, cab crushed and the steering column was forced clean through the drivers seat. Always makes me think that whatever precautions you may or may not take in terms of minimising damage, you're only really playing the odds anyway, every off/accident is different.
So I'll concentrate on staying on (complete with my backpack) rather than get overly concerned about what may happen if I do come off thanks.