I don't understand. So if they are no good to stop bullets why do the police and SWAT wear them?
Because they'll cushion most pistol rounds enough to avoid major damage, which is worth a great deal on its own.
Kn's point is that the term bullet proof is false. High powered rifle rounds will go through, and the bullets they do stop still hit with enough force that its like taking a sledgehammer to the chest. Its going to be painful, and you're going down on the ground.
It sure as heck beats the alternative, but a more accurate term would be bullet resistant.