I have three comments.
First, if you are Doing It Right, you and your buddy have practiced OOA drills, you know your air consumption and have planned accordingly on the surface, you are observing the rule of thirds and you are, in general, prepared for an OOA situation. Your backup is sitting three inches from you mouth and you can reach it easily, even if your primary is ripped out of your mouth.
Second, the fact that your primary hose is routed around your neck does not alter the fact that you donate your primary. Our so-called "S" drills do involve the panicked diver situation in which the primary is grabbed without warning. I simply duck my head to clear the hose (it still routes under my right arm), which puts my backup right at my lips. Even if the diver were to head for the surface, I would not become entangled in my primary. (If he/she did head for the surface, I would go for his wing and dry suit inflator controls).
Third, when I pass my primary, my pinky finger is already on the purge button. Thus, the OOA diver does not get a flooded reg. If it is yanked out, then I reach over and purge it with my right hand while grabbing my backup with my left.
I am not uncomfortable with passing the primary regulator because (1) my backup is as good a regulator as my primary in terms of performance at depth; (2) I know that my backup works because I regularly clip off my primary and do the dive (or at least some of it) on the backup; and (3) I know exactly where it is located.
I'm not knocking other systems. However, IMHO, to suggest that DIR configuration somehow places you at greater risk when dealing with an OOA is not correct. In fact, the system makes sure that you pass a working regulator to the person who needs it the most while providing an immediately available backup. The same cannot be said for octo's that live happily in their rubber balls or tucked into a BC and that get used once in a great while.
