Cats generally don't ruin things, unless they aren't getting proper care or attention and training (yes, you CAN train a cat). Scratching cats often don't have enough they can scratch on, or aren't trained to scratch on. They need to scratch to keep their claws sharp to be able to climb and I think it also sheds old claw, so either providing them with a scratching post or three, or keeping their nails clipped or even using plastic claw caps, will cut down on scratching your furniture or whatnot.