Actually, if memory serves me correctly (for once), there was a situation several years ago where a transit provider ran out of bandwidth and started shunting traffic through a (better-connected) customer’s network. If filtering had been operating properly at that time (if there had been enough CPU muscle in the routers) as specified below, then this could NOT have happened – the customer’s network would have recognized the destination IP as not being within its address range, and filtered it on ingress. (Or, it could have checked the source IP, and if it wasn’t in its address range, filtered it on egress. My personal opinion is that both are necessary and desirable, for different reasons.)
-Mat Butler