There are valid reasons to spoof IP addresses (we do that at work in production). It's only bad if the spoofing "leaks" out to the Internet at large (at work,

There are valid reasons to spoof IP addresses (we do that at work in production). It's only bad if the spoofing "leaks" out to the Internet at large (at work, we know which machines are doing the spoofing, and it's a limited number of spoofed addresses, all directed towards own own equipment).