I have been using OpenHAB and MQTT for a number of years now.
The reason for using MQTT and not just a serial connection from the MySensors GW is partly the flexibility as many have stated previously here. Easy to test, using e.g. the mqtt.fx tool. If you want to know what is going on you simply connect to the MQTT stream and "snoop" on the messages.
Another reason is that I am running three geographical sites on one OH installation. OH is running on a NUC at my home. There is also an MQTT GW here. Two other My Sensors MQTT-GW are running remotely in two summer houses. The three GW connect over internet to a cloud-based MQTT broker and OH connects to the same broker. In that way I do not have to open any ports into my home network for access should I have had a local MQTT-broker at home.