When you get a boot that is too big you, your foot will twist and move inside the boot, and therefore your ankle and leg will as well.  As your foot bounces around in the boot, it can lead to black toenails (or losing your nails altogether), ankle pain, foot cramps, shin bang, etc.  And it will always be a sloppy skiing boot with little to no responsiveness, which means you have to work harder to get the same thing done.