Normally, you don't plan a route through service roads. But service roads (highway=service for the osm map) may very well be the starting point or the destination. The final road(s) into a parking lot is often a service road. Small roads inside an industrial area are also service roads, and navigating them is useful when visiting a factory. If the GPS say I am there, then I am there. So using the service roads should be possible. Giving them a really low speed is probably enough to keep Dijkstra from un-necessary service road trips.
