I think you're still missing the point. Let's say I run John Doe's chop shop. I have purchased my own /24 (192.168.1.0/24). I want redundant connectivity so I have service from Comcast and qwest. I use bgp to decide every I'm routing my IP through Comcast or qwest at any given moment so they both need to allow traffic from that IP space even though it may not be from me. X100000. So it's easier to just not filter at all.