I don't think we are downplaying it past the point of irrelevance. Slavery was a problem in the country at the time but not a cause of the war. I can't see 99.9% of the Confederate army going off to fight so that the other 0.1% can still own slaves. I can't see more than half the US army going off to fight to free slaves that they never have any contact with and could care less about.
BTW - did you know that Ninjas tipped the scale of battle at Gettysburg?
http://modernhumorist.com/mh/0303/civilninjas/