Like your other reply said, MQTT-SN can use UDP so may be a better fit in some situations. It's a shame that MQTT-SN hasn't seen wider use, but there isn't that much support at the moment. The protocol is being standardised at the moment, and there has been an MQTT to MQTT-SN gateway as part of the Eclipse Paho project for a while now. The Mosquitto project has the aim of adding support for MQTT-SN - it's just not as high a priority as other things at the moment unfortunately though. If it's something that's important to you, please get in touch.
Roger