When I carry my laptop, my biggest concern in a pannier is not vertical bouncing, those impacts are absorbed sufficiently by the slack in the straps and the bag itself. I find myself much more concerned with the bag bouncing off the rack itself in a lateral motion. This is not a common impact, but the frame of a thin computer is much less robust across the flat surface of the bottom and top. I have seen my fair share of laptops with destroyed monitors due to impacts to the back of the monitor while closed and in transport.