There is at least one good reason to - for using a 4k display at 60hz. Very few models can do this IIRC - only the newest one with the Radeon Graphics chip can.
Side note / self promo: I have the newest Macbook pro (with the aforementioned graphics chip), fully loaded which I am looking to get rid of (just dont need it as my job bought me another). Currently retails for $2700 with tax. Will sell for (name your price).