If I was in a window seat, had to wait for the rest of the row to clear out, and my backpack (with meds and laptop) was at my feet, I'd probably put it on my back because it would add zero time to the overall process. Once it's on, it feels like part of me. I'm used to maneuvering in awkward places while wearing it.
If I was in an aisle seat and it was in an overhead rack which I'd have to get it down from with dozens of people behind me, hoping to get out before the plane explodes, I wouldn't. I also wouldn't take anything that kept one of my hands occupied, no matter where it was or where I was sitting.
Problem is, it's hard to make rules - or even guidelines - that cover all the variables in a situation like this.