The cassandra-dev cluster (and the restbase-dev cluster before it), are primarily a way of staging changes to Cassandra clusters (new versions of Cassandra, JVMs, configuration changes, etc). Over time, we've also begun using it for staging services as well. It has been useful to have a staging database for services, but it is also useful for staging Cassandra changes, since we have actual workloads to test with.
The staging versions of sessionstore and echostore use cassandra-dev, as does PCS. Perhaps we could also move aqs_testing and test_spark3_loading there as well?
It is worth noting that cassandra-dev is in codfw, and that it's significantly smaller than the production cluster (less compute and storage).