This is something that tends to surprise developers early on (myself included, years ago). But plenty of people still use queue solutions like RabbitMQ without thinking it all the way through.

Unfortunately, partitioning introduces a design step that makes it a little harder to make processing generic. With RabbitMQ you just post to an exchange and let queues (ie., consumers) filter on the routing keys; if no queues have been bound, for example, messages don't go anywhere. If you want, or don't want, parallelism, you just run either multiple consumers or just one. With Kafka, you need to decide beforehand, and design the "topology" of your log carefully, not just for the producer, but for each consumer. When producers and consumers are different apps, this starts smelling like a violation of the principle of "separation of concerns".

I rather wish Kafka had a better routing mechanism, actually. I don't see any reason why it couldn't have routing keys, just like RabbitMQ.