There are quite some advantages.
- development is extremely agile: updates come regularly and in the DEV tree on an almost daily basis, you can contribute via GitHub (suggestions and code)
- quick updates for new (= “currently unsupported”) devices
- integrates with HomeAssistant und Domoticz; that way the community is huge and list of supported devices grows quickly
- you won’t be blamed for using zigbee devices that don’t comply with the zigbee standard (I would also love to live in an ideal world but if I bought some imperfect device I’d rather work around rather than throwing it out of the window because “it’s not standard, we will not support it”)
- great features like reporting, binding, group support, network maps…
Check out https://www.zigbee2mqtt.io/. Z2M has a lot of great things. Currently it’s not integrating well with openHAB but that should change in the future. Most critical issue is that it is using JSON messages which… “is not standard” and thus not supported out of the box by the openHAB MQTT binding. However for setting values there is already support for plain messages without JSON. There is also someone that is implementing Homie compatibility which would make devices auto-discoverable by PaperUI. It might not happen this month or next but I guess end of year Z2M should be able to work directly with openHAB.