You know the trick of slicing apple in two with your bare hands? Well, it works well enouhg with most apples but fails miserably with either too big or too small apples or when apple is not ripe enough.
Apples should be shaped somewhat like oranges - divided into sections along which you could slice
them easily without butchering them with knives, cleavers, scissors or something else equally cumbersome. Usually you just don't have the right equipment handy when you have the apples. Also, cutting with metal (even with stainless steel) always leaves distinct aftertaste.
Besides, apples would be much more convenient to eat. Imagine eating huge apples - you open your mouth as wide as you can yet you manage only to scratch the surface leaving ugly teethmarks all over. Or imagine how nice it'd be to be able to break slices and eat them one-by-one, you then could leave half-eaten apple lying on your desc not looking ugly as conventional half-eaten apples do.
I admit that hard round shape is something that makes apple an apple but slices would be still nice.