Green points are your route-nodes, black lines are given OSM-ways (which would actually in route-relation), red line is the route, blue line is an added OSM-way or an OSM-way which changes access-Tags.
If a mapper adds this blue way or changes the access-Tags of this way, the mapper have to add a node to route-relation somewhere in the upper arc. If he doesn't do so, he will break the route-relation, without touching any part of this relation. If the mapper uses orange point, the shortest distance between all green and orange point will be orange line, which is a broken relation, too. So better use light-green point. Ok last point could be solved with other routing-algorithm.
The main problem I see is, that mappers had never care about relations doing above things. Noone will care about a relation, if he changes an access-Tag, a highway-Tag or adding/modifying a highway.
If there are gaps in an actual route, you can spot them out very easy, because you see this gap. In a node-based model, there wont be a gap. Only if there is no connection between two nodes. If I change a route-node or add OSM-ways (see above) the route will just change the route. You can only figure out the fault, if you compare it to reality.
In my example (picture) its quite easy to detect, that you are breaking a relation. But reality is in many cases harder.
In my opinion it's better to give editor more AI, that he can assist the mapper in handling ways with many relations.
Henning
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