I don't disagree. Do you have tools for reliably detecting unacceptable default route usage? Until those tools are available and until IXP's agree to police their interconnects, the MAC filtering approach is the only real solution available.
I've long thought that the routers should be able to reject packets which come from MAC addresses to whom no route has been offered for the destination. Tony and Jerry and Andrew and Hank and Joe have all explained to me why the current Cisco design makes this impossible, but I can dream. (Perhaps Cisco's hopeful competitors will take this as a product requirement.) But let me turn it around. With no means of detection, why do we suspect that it's a problem? That is, why doesn't the cause for suspicion also work as a means of detection?