The licensing cost of Zing generally makes this a bad trade-off. It's much cheaper to purchase more hardware. Zing is targeted at vertically scaling very large JVM heaps, where it's valuable to have massive amounts of data on a single, big machine.
As an ex-Azul employee I can say there's a good number of Azul clients using a Zing+Cassandra setup, so the price point is right for some people at the very least. Zing licence cost has also changed in recent years (3.5k per server last I looked, and that is before you haggle some bulk deal) so not sure if your impression is calibrated to that new price point.