Before I managed this server (and the Wiki), I had a fairly dismissive take on spam - it was a pain, but deleting/filtering it on the client side (email) wasn't that bad). At the server level though, it's a whole new ballgame. For one thing, it's negative PR - it looks bad. For another, it can get you Google Squashed:

That was not the end of it, however. A site like Javalobby is constantly being spidered by the major search engines, so they were quick to pick up all of these new trash messages. It had been aggravating to spend holiday time cleaning up the unwanted mess left by our guests, but the real problem didn't surface until we started going through our normal morning routine yesterday, having just returned to work from our holiday break. We generally take a look at a variety of statistics in the morning before proceeding into whatever development work we're doing. Having been out of the office for almost two weeks, we had a lot of stats to look at. It took no time to see that something was wrong - traffic was down. A little more investigation revealed the problem.

We had completely disappeared from Google's main index! If you run a website, then you know how serious a problem this is. On any given day over 10,000 visitors arrive at Javalobby as a result of Google searches, and suddenly they stopped coming! We had apparently been grouped together with the spammer's viagra and casino sites, and poof! Suddenly we no longer existed in the eyes of Google, the world's largest search engine. Countless thousands of well-ranked pages gone in a blink. Perhaps you now understand why I would commit a violent crime if I caught those forum spammers? In essence, they have wiped out strategic positioning that we took years to build.

Lots of patient work at building a community can just go up in flames. Looking at my referer stats, I see the same thing - much of my traffic comes in from Google as well. Monitoring spam on a public site is just one more thing you have to deal with - if you let it go, you land in oblivion.