The principle of kairos, however, goes deeper than 'mere' rhetoric. It has to do with an important principle of timing. The use of kairos requires that the writer and/or orator pays attention to his or her audience and understands precisely when the right word would make all the difference in the outcome of the piece of writing. It requires an ethical awareness of others' needs, at the same time that it relies on the orator's sensitivity to others.
So kairos is ultimately an ethical consideration, and the reason this is important is because rhetoric is usually seen as necessarily unethical. Rhetoric is considered to be only interested in persuasion, in manipulating the listener into doing something they would otherwise never do. This is a perception about rhetoric that never really dies, and is only made worse by the frequent misuse of the word 'rhetoric,' which has come to be associated with the worst, most avaricious aspects of politics.
Relying on kairos, however, indicates that you understand the use of moderation and balance in the speech act. Think about the times you've seen or heard someone who waited for the precisely right moment before they introduced a new topic to you, or told you some piece of information that was, perhaps, upsetting. Their sensitivity to your needs told you a great deal about their concern for your welfare, proving their integrity and self-control. The same is true for those giving a speech or writing a piece of persuasive argumentation for their audience, and that awareness of correct timing is a powerful tool when constructing an argument.