>>> What’s the worst thing a spammer can do if they sign up? <<<

Depends on your size and resources. Imagine you're basically scrappy with a very small and tight budget and still trying to validate your idea, and you're using one of those providers that gives you a free quota (like Google App Engine), you don't want 'spammers' to drain your free allotment of resources. I know someone who has a small niche blog in the health sector whose blog was repeatedly targeted by spammers/bots. He repeatedly saw increases in his bills till he had someone audit his blog and try to block the bots