Worn tyres: With the worn shock not being able to keep the tyre in contact with the road, it bounces on the road causing worn flat spots on the tyre which reduces the life of the tyre by a huge margin - expensive.
Poor Braking: When a car brakes with worn shocks the tyres tend to bounce causing the tyre to loose contact with the road and extends the stopping distance by an extra 2.6m from 80km/h in the dry ! This could be the difference between hitting someone or not. ABS braking systems are even more affected.
Aquaplaning: Normally this happens when there is some water on the road and your car's tyres are worn, as you go over the film of water the tyre lifts off the road and you have complete loss of control. The same thing can happen if you have worn shocks - Even if your tyres are in good condition!