Who uses a gray background anyways?
one of the first things you learn in art school is that balancing images (digital or analog) in an appropriately lit setting is crucial. photo labs and digital photo labs have neutral lighting, and it's ideal for walls in these rooms to be medium grey. additionally, it's extremely common for professionals to have their desktops set to medium grey as it's a neutral setting that allows them to optimally balance digital images.
finally, iMacs with screens that have color gradients or are unevenly backlit are impossible to calibrate correctly.