MQTT can be quite handy in many use cases. The issue here is how best to implement.
Case 1. Let’s say for example, you wish to contribute NO and NO2 data to an open data set (many public bodies already do this) as your imp device is already capturing this data for some other reason. Let say you have an option to publish your data to the public broker handling this data set via MQTT. Typically the broker does get to “see” where the published data has come from. I would say in this scenario you may not want to provide visibility from your imp agent or device so in this case I would want to use a peripheral device / module attached to an imp device to do this for me. This way you have at least hidden your imp network from a public mqtt broker.
Case 2. Let’s say you are using imps attached to water temperature sensors at a local public swimming baths. You decide to make this data public. MQTT is probably the easiest method to implement so decide to set up your own broker service where the public can subscribe to to get the temperature data feed. In this case I see no problem for imps to publish data directly to this broker as you are in control of this broker. The question then, assuming it were possible, is whether it is simpler to do via imp device if broker is local or only handle via agent.
Case 3. Let’s say you want to subscribe to some public data set which is offered via MQTT as you need to use this data with your imp application. In this case you would want your agent to do this as it is best placed to handle the data security side.
Case 4. You want to capture a huge data set which is being sampled at high frequency but you don’t want to / aren’t able to store at device level. I believe MQTT offers the best method to offload this data (as in publish it) elsewhere into better infrastructure that can store this data for you i.e. a broker.
Maybe there are other scenarios that @hugo and others have considered or maybe there are other ways to implement my case examples. Would be interested too to learn other opinions.