To clarify my thinking on this topic, I figure that potential oxygen contact is one big reason that argues against using a secondary. In spite of reassurances to the contrary, my secondary usually does not have enough dissolved co2 to bubble the airlock after making the transfer. To me this potentially makes the secondary the major bad actor re. o2 contact. (Primary is obviously no problem. o2 in the bottle headspace should be removed by aerobic fermentation of the priming sugar.) If I had a co2 bottle, I might use that to purge the secondary headspace, but I figure that the sugar does a better job of purging any dissolved o2 introduce when transferring from the primary. Because I dry hop more often than not, adding the sugar is a pretty trivial amount of extra effort. Besides, it helps me relax and not worry.