It isn't just useless, it is a bad design too.
(fast translation, from my book in spanish).
Even when nobody admits it in public, the real reason about why the OSI model has 7 layers is that by the time it was designed, IBM had a 7 layer patented protocol called SNA (systems network architecture). By that time, IBM dominated the computing industry at such degree that everybody feared that IBM used its market force to push everybody into using SNA, which could change anytime they wanted. What OSI pretended was to create a reference model and stack protocol similar to that of IBM so that it could become a worldwide standard not controlled by one company but by a neutral organization, the ISO
(fast translation, from my book in spanish).