It seems to me that compared to your phone, a power brick dangling off a charging cable is much more likely to slip off your lap unnoticed and get wedged in the seat hinge only to get subsequently punctured.

I recently took a flight where I had a laptop, my phone, a power brick, a new power brick for my wife, a second phone (for reasons) and a battery for a piece of ham radio equipment in my backpack. As I got on the plane, I was thinking I was probably one of the risker passengers on board :) Anyway, when I use the brick, I keep it zipped in a jacket pocket with just the charing cable coming out in an effort to keep it from finding its way to a place that it shouldn't.