Plain Language Isn't Always the Answer
If you want to reach the widest possible audience, then you need to speak as plainly as possible. That sounds pretty intuitive. After all, if your audience can't understand, how are they supposed to get on board with you? That makes a great deal of sense, but it's an incomplete picture. Here are just two scenarios when it pays to use jargon.
Storming the Castle: When Experts are the Target
Sometimes you are trying to change the thinking of a discipline or profession. Their technical terms are their currency and by changing (through use) those terms, you are able to mobilize those groups and institutions in a way that plain language just can't. For example, it's one thing to convince the public that meat is murder and quite another to convince a judge meat is homicide. They both may help you advance your cause but they are not interchangeable.
Elevating the Commons: The Dignity of Technicality
Sometimes the absence of a technical label for something is because experts do not take it seriously, pay much attention to it, or understand it. This lack of seriousness may translate to popular sentiment. Once something seems to be a matter of expertise, others tend to take it more seriously as well. In these cases, labeling something in a technical way when addressing the public may be vital to gaining the respect and attention you need, whether or not you have yet succeeded in convincing experts. This often comes up in health advocacy. It's one thing to talk about someone being sad a lot and quite another to talk about clinical depression. It's not just that the first is an oversimplification of the second (which of course it is), it's that the second imports a whole world of medicine and science to imply that certain types of causes are at play and certain types of responses are appropriate.
Don't get me wrong. This is not an excuse for leaving in that jargon in your next report because you couldn't think of a simpler way to say it. Language imposes a cost on the audience. They only have so much time and understanding to spend on what you have to say. But when it comes to your linguistic budget don't save money for the sake of it. Some advocacy objectives require using $5 words. Know when to use them.