All of my lightbulbs, occupancy sensors, etc just connect directly to WiFi, and run custom firmware that I wrote so I know exactly what they're doing and how to control them. They make no attempt to access the wider Internet, but they're all on a vlan without Internet access anyway.
It feels like introducing Zigbee to this would just be an extra hub device taking up space, acting as an extra point of failure, and making it more complicated to develop against my devices. As it stands now I can easily manually control devices by piping crap into netcat if I need to for some reason, since they're all just normal IP networked devices. I think I would have to jump though extra hoops to do similar things with Zigbee.
Is the main aspect driving people to Zigbee just that off the shelf consumer smart devices that use WiFi tend to be annoying dogshit, and Zigbee keeps manufacturers in line better? I don't see any reliability or simplicity benefits to it, just the market poisoning WiFi and Zigbee being the only worthwhile alternative.