Beyond freeing Southern slaves, the Emancipation Proclamation also permitted the enlistment of black soldiers in the Union Army. By the end of the war, more than 180,000 blacks had served, composing roughly 10% of Union forces in the field. That is a hugely important manpower boost, if you are looking for a material impact.

Another material impact would be the withdrawal of England from serious consideration of intervention in the war. Since their slaves had already been freed and the moral issue had been settled, support for intervention dropped off sharply among British commoners. While some British newspapers and upper-classmen argued along your lines regarding the originality, morality, and effectiveness of the proposition, there were also 56 public meetings held throughout the winter of 1862-3 in which resolutions were passed in praise of the Proclamation. A potentially decisive war effort was short-circuited by the prevailing sentiment of the working classes.

The realpolitik elements of the Proclamation are, in my opinion, quite overblown. Lincoln was ardently anti-slavery. The areas of the country in which slavery was preserved were miniscule in comparison to the territories being freed. And I don't think that many people expected the remaining tatters of slavery to be preserved perpetually while the main body was destroyed. Missouri, which was exempt from the proclamation, nevertheless swiftly instituted a plan for gradual emancipation.

The Emancipation Proclamation is not a historical footnote because history is not inevitable. This specific event was the beginning of the end of America's original sin. It was a complex document, a product of specific circumstances, and it was the boldest political blow ever struck against slavery and for civil rights in the history of the United States.

Sources:

The Emancipation Proclamation by John Hope Franklin The Emancipation Proclamation on civilwar.org