What is Rhetoric and is it useful? Aristotle defines Rhetoric, in his work by the same name, as follows: "Rhetoric may be defined as the faculty of observing in any given case the available means of persuasion" - Aristotle, Rhetoric, book 1, ch. 2, 1355b26 -
As such, we can affirm the following: POOR rhetoric is no substitute for critical thinking or sound arguments. On the other hand, GOOD rhetoric is the application of critical thinking and sound arguments to the convincing of one's interlocutors.
In the Rhetoric, cited above, Aristotle explains that there are 3 elements of rhetoric: ethos, pathos and logos. That is, the character of the person speaking, the appeal to the passions, and the argument presented.
The above definition may be separated into 4 sections for comment: (1) the faculty of observing, (2) in any given case, (3) the available means, (4) of persuasion.
Now, each part, is contingent on the notion of seeking to persuade (4). That is, the purpose of rhetoric is to convince the audience (whether it be through the written or spoken word) of that which the speaker or writer is seeking to prove (this could be anything: that Jesus is God, that abortion is wrong, that krispy kreme makes the best donuts ever, that guiness is the best beer, etc.). The art of rhetoric (the faculty of observing - 1) is possessed by he/she who knows how to convince any audience that he/she is presented with.
The two questions that are more important are 2 and 3. When we say, "in any given case", the idea is "regardless of the audience". The idea is that the true rhetorician knows how to read his audience, to see what it would take to convince them, and is able to use the means that are available (3) to convince them. The audience could be a motorcycle gang, the ladies breakfast group at some baptist church, a group of Catholic priests, a close friend or relative, one's wife or husband, or the humanists society. This is what is meant by "in any given case".
"In any given case" goes closely together with (3) "the available means". The available means are the 3 elements mentioned above (ethos, pathos and logos). They are applied differently depending on the "given case". So, for example, you are walking home with your wife and child after eating at some pizza parlour in Québec City, and you are accosted by a man with a knife. The character (ethos) and appeal to the passions (pathos) and arguments that you use (logos) to convince him not to kill your family will be quite different from that which you would use if you were trying to convince the ladies breakfast group at some baptist church of the importance of apologetics.
In the first situation you might try a number of different strategies: (1) you might try to make yourself look mean (ethos), and try to scare (pathos) off the would-be robber and murderer, by threatening him (logos) with potential death at your very able hands; or (2) you might try to make yourself look poor (ethos), and make him feel sorry for you (pathos) by telling him (logos) that you are a poor PhD student. In the second situation you might dress up, thus presenting yourself as a scholar and representative of some church group(ethos), you would speak to them of the dangers (youth leaving the church) of ignoring apologetics (pathos), and you would explain your point in a way that they understand (logos).
From a christian perspective the idea of ethos is not that you create a false character, but that you are in constant pursuit of the image of Christ, such that your character is evident to all men. (also, check out 1 Corinthians 9:16-23 where Paul says exactly what I have been explaining about preaching the gospel).
From a christian perspective the idea of pathos is not to create false feelings in the person, but to appeal to their conscience (for example, some might use pictures of abortion to appeal to the conscience of people).
From a christian perspective the idea of logos is to always preach the truth. This does not mean that we always present our positions in neat deductive syllogisms, but that we tailor our presentation to the crowd. So, in Acts 17:1-6 Paul begins his defense of the gospel, with the Jews, in the Old Testament. But, in Acts 17:16-34 Paul begins his preaching of the gospel, with the greek philosophers, by appealing to God's providence.
So (and this should help with what you are wondering about above, as well as clarify, even more, my qualification of your statement), "the available means" never means (for a Christian) to lie, or resort to brute force, etc. POOR rhetoric will resort to lies, brute force, promising things that the speaker knows he/she will never do, etc. The properly moral use of rhetoric understands that "the available means" do not include "immoral means".