In the case of signature spam we have to think, "Well what would spammers do instead?", "What is the impact to normal users?", and "How often do we have to cleanup this mess and how troublesome is it?". We've even read through documentation on spam bots that you can purchase to see what the software's reaction typically is to this.
Overall I think it was found that the spammers would adjust (so little to no impact to them), new users might get frustrated, and if they didn't put spam links in their signatures, they'd put them in their posts. I actually get more frustrated cleaning up people that quote spam than I do cleaning up the actual spam although it's fairly rare.
I know sushi's post a bit up there quoting Blue Velvet's fake spam was in jest, but if Blue Velvet had been a spammer and then sushi quoted it 2 pages later in the thread saying something like "OMG Spam!", the odds of us finding the quote of it is fairly slim. Now if BV had had the link in the signature area, sushi wouldn't have quoted it and while the quote might remain, the link would not.
Although I'm sure I've done the existing thread reply thing, I more meant the fresh threads started by a spammer where they've gone and advertised something so stupid I can't help but call it what it is. As a general rule I just report spam, sometimes with content intended to amuse the mod(s) who finds it.
