Body armor mostly stops right above the belt line. Take a pelvis shot. Very disabling and very fatal due to the large blood vessels and bones. Bigger target, doesn't bob and weave around like someone's head when they are moving. Also the downward angle of your shot is less likely to cause collateral damage if you miss.
+1, I couldn't agree more. The key is to practice precision shot placement so that you can relocate your shots to the pelvis or head when a body shot fails to stop a threat.