Opossums playing and eating with my cats outside?

There's an opossum that comes out in my back yard every night, and the opossum will eat with my cats (both their heads in the food bowl at once) will lay day beside my cats, and occasionally plays with them (chase each other pounce on each other, and I'm pretty sure they are not hurting each other.) My mom thinks they are disgusting diseased oversized rodents, and always tries to shoot at them with bb guns. Should we be shooing them off? What harm could they cause?