Like all of the Maxims, this one is simple in theory, but difficult in practice at times. Simply put it is rude to talk about someone behind their back and even ruder to talk about their positions behind their back when they cannot defend themselves. If you are going to talk about someone, you need to do it when they can respond directly to you in a proper conversation or debate. Otherwise, you run the risk of only telling part of the story to the audience. This can lead to confusion on the part of the listener, and that confusion is something that needs to be avoided if at all possible.
This Maxim, unfortunately, is often completely ignored in politics and other fields which seek to shape public opinion. We are confronted by people who tell us that their opponent (whoever that may be) is one way when they may be entirely different from how they are being portrayed. This can lead to a completely unnecessary polarization in opinion. And the polarization is something that we need to avoid if we are to be in keeping with the message of “Nothing to Excess” as well as the message of this particular Maxim.