Why not just set everything into essentially an S3 bucket, and then process off of it. What's the benefit of a broker in between them really?
Dumping everything into an S3 bucket may be perfectly fine and even efficient for some projects. I personally don't see myself wanting to use S3 buckets for transmitting messages between multiple different systems with different purposes.
If you have a lot of different services producing data that multiple services will consume, then a pub/sub system can simplify things. Some systems, like Kafka, also have added redundancy/scaling benefits.
Currently this is done for the control plane by just loading them directly into Elasticsearch. Increasing the availability for all users, the consideration is, does a pub/sub add anything that just throwing the data directly into an s3 like bucket (swift in our case).
I just assumed it did. Everyone does it. Scale, decoupling the sender from the receiver...
But... I mean, why? What's the point of an additional hop, and another thing to manage. If your source is already wrapping metadata, and perhaps putting it into a structure, what is the value of a pub/sub there? If you can load it to the pub/sub, you can load it to the s3 bucket in the same manner.