I like the low wind drag on I-70, and yeah, less branch/overhang damage potential. I also think it's easier on the bikes to be suspended by their frames, compared to clamped to their forks or something, I've never liked the side load bikes on top of the truck are subjected to.
Plus on the rack I'll have a Thule box, jack standard, 10x10, grill, have the awning on the side.. I got enough on the rack.
edit: and what are you doing on the computer anyway Corbet, don't you have a barrier to build?