Now and then the RFID reader can't read my Nexus card, and I can assure you they want to see a passport at that point.
Actually not (I don't think), since they don't require a passport for land crossings yet.
But it's a good point, if they (both US and Canadian governments) continue to promote the Nexus card as an acceptable secure document for WHTI instead of a passport, how will they handle that situation? I'm sure many people will sign up for Nexus instead of getting a passport (the same people who never had a passport for entering the US before).
They did tell us that if the iris scanner doesn't work, we can go to the crew/diplomatic line with our card, but that was the Canadian agent since the US machines weren't set up in YUL at the time.
=aw