There are two very different reasons to cluster:
Its important not to confuse the two goals. Note that a reliable cluster will be LESS scalable and performant than even a single broker - replication is extra work on top of normal processing. There is also:
It's not clear where to draw the line between federation and clustering for scalability.
Reliability clustering is orthogonal to scalability clustering/federation, which means they can be combined.
Just replace the individual brokers in your federation or scalability cluster with reliable broker clusters
and you have a reliable and scalable system.