Well, the one use case that is not covered by simply reporting everything as soon as it  happens is Pau's request to track the time spent in failed conversion steps, from entering the step to closing the window/navigating away. To do that, we would need a leave event which is either saved when the user leaves (but an asynchronous request would be lost much of the time, and a synchronous one would decrease site performance), or store the event and replay it as soon as the user is on a wiki page again. I guess we can just use localstorage for that; with the flow_id multiple tabs is not a problem, and since we only lose the last event when the logging fails, not the whole event chain, we don't have to worry about the results becoming biased by non-localStorage-supporting browsers or infrequent users.