They do this in all locations when there is nobody in the NEXUS line and things are getting backed up. However, this means there are usually a couple of "amateurs" ahead of any NEXUS newcomer. The major advantage of the NEXUS line is to fast track the main queue, just as in the US where the elite fast tracks get you to the document checker, but then into the normal queues for the security process. From their viewpoint, they cannot afford to have one lane under utilized and empty while others are working full tilt. This tends to get worse in the summer months (like from now) because the number of leisure travellers is greater and queues move slower.