It is because they ahve a bad wear pattern. My tires did the same thing when I bought my car. I had them rotated and it was fine. It will mess up greatly the handling of the car and might throw the car out of alignment. If it isn't that bad and you haven't been driving with them like that for awhile then the other things should be fine. The motormounts should be fine, although excessive shaking of the car has been known to loosen or destroy the mounts.
Dan
