[psfried]

IANAE on Flink, especially when it comes to the internals. But I think that the decomposition of computations into distinct map and reduce functions seems to afford a bit more flexibility, since it can be useful to apply reductions separately from map functions, and vice versa. For example, you could roll up updates to entities over time just with a reduce function, and you could easily do so eagerly (when the data is ingested) or lazily (when the data is materialized into an external system). That type of flexibility is important when you want a realtime data platform that needs to serve a broad range of use cases.