I'm a bit wary of overlooking the expressiveness argument. It's really easy to miss important cases and then those use cases end up high and dry with a non-solution. As an example, I'm working on a use case right now where registration context would be a huge problem - it's (again) a tracing use case: our data service allows registering interceptors for incoming data and the interceptors are what actually trigger the UI update (possibly via a signal reaction, which is another callback registration that would need to preserve call-time semantics instead of registration semantics for this all to work). If either the RPC interceptors or the signal reactions use registration context, we're sunk and there's no way to trace an interaction from the initiating event all the way to the UI update, given how loosely-coupled all the reactions in the framework are.