Before I installed Mollom on my sites, spam was a continual threat.  Not only were email forms at risk of being exploited, flooding their recipients with hundreds of unwanted messages a day, but comment spam could take over a site, covering useful pages in garbage or actually coopting the browser with iframes and other dangerous HTML tags.  I fought back as best I could with captchas and input filters, but it was an arms race I wasn't likely to win.