Isaiah 40:4-5 (New International Version) - - - 4 Every valley shall be raised up, every mountain and hill made low; the rough ground shall become level, the rugged places a plain. 5 And the glory of the LORD will be revealed, and all mankind together will see it. For the mouth of the LORD has spoken." Martin Luther King - "I have a dream that one day every valley shall be exalted, every hill and mountain shall be made low, the rough places will be made plain, and the crooked places will be made straight, and the glory of the Lord shall be revealed, and all flesh shall see it together."
Here is my opinion: if you going to quote scripture, in all its wisdom and power (especially if you are a minister), then cite it.
To take God's incredible words, and act like they are your own, is a terrible thing to do.
In King’s defense, he was coming from an environment wherein one would not just expect but have utter confidence that people really knew their bible. Southern Protestants make a point of not just reading the Bible but actually knowing it, and so the reference would be quite plain.
Devout black Christians are no different. So, to accuse King of plagiarism for weaving Biblical references into his public speaking is very unfair. There is plenty upon which he can be fairly and honestly criticized, such as his ties to communists, but this is not fair criticism.