Question: From Obama's second inaugural speech:

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> We, the people, declare today that the most evident of truths – that all of us are created equal – is the star that guides us still; just as it guided our forebears through Seneca Falls, and Selma, and Stonewall; just as it guided all those men and women, sung and unsung, who left footprints along this great Mall, to hear a preacher say that we cannot walk alone; to hear **a King** proclaim that our individual freedom is inextricably bound to the freedom of every soul on Earth.
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I know that he was referring to Martin Luther King, Jr. But why did he use the indefinite article "a" before "King"? Is it better, for example, to use "Dr. King" or just "King" instead? Does the use of the word "a King" provide a sense that Dr. King was the king of something, such as righteousness, justice, etc?

Answer: 
Stylistically, the phrase "to hear a King proclaim..." echoes the previous construction "to hear a preacher say...." The repetition of the indefinite article maintains the flow of the speech. (Imagine how awkward it would have sounded if, in place of "to hear a King proclaim," Obama had said "to hear the Reverend Dr. Martin Luther King, Jr. proclaim....") Of course, the construction also invites the listener to consider the double-meaning of "King"--as the other answers say, it identifies the speaker (MLK) and also the speaker's symbolic importance (as a leader of the Civil Rights movement and as the "moral leader of our nation").
It is a pun being used stylistically. MLK is likened unto a king, and King happens to be his name.