As a speech teacher, I must agree with your first poster, that the "I have a Dream" speech is not only effective, it is a classic example of persuasive speech format.
Under Monroe's Motivated Sequence (which is the persuasive speech format I teach), the first part of a persuasive speech should always include a "Hey" or "Hey, pay attention" (an attention-getting device).
What better way to get an audience's attention, then by basically telling America that it welshed on a promise to the blacks. That a promise made by our forefathers was not kept. Anytime you start a speech with a startling statement, or a description of a big conflict or problem...you've captured the attention of most of those listening.
Let's say you were an American in 1963 who thought great strides had been made in the civil rights movement, and then Dr. King says the following near the beginning of his speech:
But one hundred years later, we must face the tragic fact that the Negro is still not free. One hundred years later, the life of the Negro is still sadly crippled by the manacles of segregation and the chains of discrimination. One hundred years later, the Negro lives on a lonely island of poverty in the midst of a vast ocean of material prosperity. One hundred years later, the Negro is still languishing in the corners of American society and finds himself an exile in his own land. So we have come here today to dramatize an appalling condition.
That statement would certainly make me rethink my position...and so it did for many in the crowd that day.