Ok, I can think straight up of one huge use case. With 64 bit integers, I can blend all four colour channels of a pixel in one hit. For graphics operations this is a huge performance boost. (For those unfamiliar with the concept, you allocate 16 bits of the integer to each of rgba, which are only 8 bit values. The extra 8 bits per channel allows you to handle overflow on operations, which means you can do an op in all four channels simultaneously).