There is some expansion and contraction that goes on and not having any spacing can cause the touching edges to lift up or press down as they push against each other. This pressure can also cause some breakdown of the OSB material typically along the touching edges.

This can cause a wavy look to the roof surfaces but I personally have never seen anything I would call a failure of the roofing system due to a lack of spacing between the sheets of decking. Not saying its not possible, just saying that I have never personally seen it.