When you generate your own route on a computer and use it as a file in The Trip Planner of your Nuvi, you are using the Nuvi in Mulitpoint Routing mode. In auto routing mode there are only start and destination points, with all other roads in between being auto selected by the nuvi, there is only one priority I.e., the destination
When in mulipoint routing mode, every waypoint in the route is of equal priority, so if one is missed, it is still a destination that has to be reached, what is needed is a single destination with top priotity and all 'via' points being given a lower priority, however I don't think it works that way
I think all Garmin units will handle mulitpoint routing in the same way
EDIT
I have looked into this further, and it looks like it makes a difference how the route is created in Basecamp, If the route is made up of pre-defined Waypoints (Blue Flag), then these points must be visited, however if you create the route by clicking on map features you create a shaping point (Black Dot), this item in a route has a switchable option, e.g. Alert on arrival = On /Off, I'm guessing that 'Blue Flag' has the same priority as the destination, (e.g. must be visited), and a 'Black Dot' set to Alert on Arrival = off has a lower priority and does not have to be visited