Malcolm X was more interested in a separate nation for Blacks rather than Civil Rights. His famous "by any means necessary" speech in which he advocated the killing of Caucasians to obtain the goals of his group certainly was not a call to unite the nation. His radical ideals of black identity, integrity, and independence challenged King's dream of equal opportunity.
Having a background as a youth that was less than stellar--street hustler, drug dealer, and leader of a gang of thieves in Roxbury and Harlem, imprisoned for robbery from 1946-1952, joining the Nation of Islam and changing his surname Little to X as a rejection of white slave names--did not lend him the credibility with other races that the Reverend Martin Luther King had, either.
The previous post's thoughts on whether or not "essential" is the most accurate descriptor is a valid one. I think that Dr. King's primary message was something that allowed more Americans to accept the plausibility and need for Civil Rights for African- Americans. Dr. King's manner of framing the movement as something that was driven by an acknowledgement of human rights, making the political struggle a moral one. For example, in the "I Have a Dream" speech, civil rights for African- Americans was seen in a moral light, an ethical and spiritual responsibility for all to acknowledge. Certainly, other leaders argued the same elements, but Dr. King was seen as the spiritual figurehead for the movement, the individual with whom so much morality was associated. In this light, he was able to bring moderate and liberal white Americans to a struggle that was largely seen as solely a black problem.
That being said, I don't think one can discount the impact of nationalistic leaders like Malcolm X. While his vision caused a great deal of fear and consternation in the same people that felt comfortable with King's, I think that this was deliberate. Malcolm X's stance and deliberate calls helped to bring to light the idea that if white Americans were not comfortable with Dr. King's solutions, there were other approaches that might be more threatening than anything else. I think that later on in his philosophy Malcolm X understood that one of his functions was to get more people to embrace King's vision specifically because what he was articulating was deemed as so "radical" for the time period. In the oddest of ways, acting as an occasional foil helped to bring the two men closer together and show alliances while others sought to create division.
I guess I am not sure what you mean by essential, but he was a more successful civil rights leader because of his ability to appeal to white Americans.
Although the Civil Rights Movement was driven by the efforts of black people, the movement could only succeed with white support. This is a basic truth in a country that, in those days, was more than 80% white.
Martin Luther King appealed to moderate whites. His message of nonviolence and universal brotherhood was an attractive message. He was a Christian minister, educated and middle class.
Malcolm X was an ex-convict who was, especially early on, anti-white. He was a black nationalist, uneducated, and Muslim.
Because of their different backgrounds and different attitudes, King could be much more persuasive to white audiences.