That was my thought, too, although I really can't justify it. If a pannier is solidly attached, I don't see how it's better or worse than sitting the laptop on top of the rack. I don't regularly travel with a laptop, but when I do I opt for wearing it in a satchel or a backpack, because I figure I provide some shock absorption that the rack/bike will not.

And to echo what slcbob said, your rack has to have the 2nd, lower set of rails to use any wide trunk with panniers. I have the standard MTX trunk bag, and I have tried to use it with panniers of a higher capacity than the zip down panniers built into the trunk bag, but it doesn't work. The bag is too wide and interferes with the panniers. That laptop bag looks considerably wider than bags, so I expect you'd have the same problem.