Hi Marcus,
Usage scenario 2: Reference counting.
the main problem with reference counting is that the clients have to explicitly release the reference. Thus cooperation is needed, since L4 do not send a notification if an object e.g. a task is destroyed...
Situation: S -> C -> (1 reference) A -> B
Goal: /-> (1 reference) A S-> C -> (1 reference) B
In your scenario both clients A and B have to cooperate with C for the release notification. Since both cooperate anyway, the client A could easily ask C to map a new reference to B which makes the map-lookup unnecessary...
Bernhard