While we shouldn't have to, it seems the hubs get bogged down easily, and sometimes it helps to split the load between multiple.
For me, I use the radios on the C5, and all quick-response automations. Lighting, door knocks, etc. I use my older C4 with the radio removed to handle things like endpoints, Chromecast Integration, follow me setups, and all 'complex' automations.
Though I would argue that if my hubs are sharing most of the devices via hubconnect that it's probably creating a lot of overhead, and a single, more-powerful hub would be better overall. If they released a "beast mode" hub, I'd probably pick that up.