Why is it considered bad to switch a dog's food?

Is that just for kibble dogs? My dog eats homemade and mostly raw and hardly ever eats exactly the same combination twice and never has an issue. She can (and has) eaten a different protein from bison to beef to poultry to fish and eggs every day in a week without so much as a fart. Even my teenage cats get a wide variety without a problem. So is this just an urban myth or what?