Given the fact that racial disenfranchisement and discord was so widely present in America, I think that Dr. King had to frame his speech in the form of a dream. He offers a conception of what can be which is in very high contrast to what is. This presentation had to be in the form of a dream to display how far Americans as both a people and an entire nation had to move in order to see the day when race is not an obstacle to opportunity and individual progress. Given the fact that the speech became one of the hallmark moments of the Civil Rights Movement, I am not sure if there were any such losses because it helped to conceive a world of what should be. This expression of the conditional is one where King was able to ascertain a level of moral superiority to those who stood in opposition of progress on the issue of Civil Rights.
The benefits King derives from posing his vision of future racial harmony and equality in the United States as a dream, is that he can describe and paint it as an ideal, as something beautiful, something people would want to achieve and believe in. In order to achieve it, he knew the population had to visualize the possibilities - had to visualize the dream.
But dreams have other connotations - that which is unattainable, something that is too ideal to ever become reality. If he only poses the dream without giving direction on how it can be accomplished, then the vision he paints in his speech never gets off the canvas and into real life. That was the risk and the potential loss from the speech.