Pick anything - a paperclip; SOMEONE has a story about it killing someone. Regardless where it is, 100 lbs of anything can kill you in a vehicle wreck. It's at least as likely to kill/injure an occupant from the bed as from a seatbelt or floorboard.

 

If you're dumb enough to set in on a bed cover, sure, it's coming through the rear window.

As most of us place things IN the bed rather than above it, I believe it's safe to say your statement is wrong. In most wrecks vehicles remain upright and the contents of the bed may strike the bed, but remain contained. In violent wrecks objects can go flying after striking the bed, but most of the energy is absorbed by the impact with the bed, and the contents generally move in the same direction and at the same speed as the vehicle. In rollovers, spins, and multiple impact wrecks, all bets are obviously off.

Things outside the cab tend to stay outside the cab, so it can't be said that something that is in the bed is as likely to kill and occupant as something inside the cab. What can be said is that something outside the cab that enters the cab is as likely to kill an occupant as something that is already inside the cab.

A 100lb propane canister (15x48?) secured vertically against the cab side of the bed should expend almost all of it's energy against the bed. Secure it further away so it can wrench itself out of the mounting bracket, you introduce a chance that it will come wholly through the glass.

 

would you consider rings in the e-track mounts appreciably less secure than bull rings in the stake pockets?

 

If they're installed properly, and used correctly, they'd be MORE-secure. It's better to hold the cargo DOWN to the floor, rather than UP to the bedrails.