I've noticed the same thing with Metroguide 5. I can calculate a turn-by-turn route with directions in Mapsource, but then when I transfer it to my 60c, it just uploads my main waypoints and uses the 60c to recalculate the route. But without City Select, I think it only uses the basemap for the auto-routing. That's why you're ending up with a route that is fine for highways, but not for sideroads.
It's too bad it won't transfer the route with directions that is calculated in Mapsource.