As soon as you start letting users participate and interact with your site you have a problem with spam - and then you need to spend time moderating the content. This takes time you do not really have.

Mollom is supposed to help with this:

If you spend a lot of your time eliminating comment spam, profanity, flaming by troll users, robot-created content and other poor-quality contributions from your moderation queue, you won’t have enough time to complete that great new feature or engage in the meaningful dialogue that can make your web community a success.

The Mollom service helps evaluate content quality, stop spam and lets you concentrate on building and improving your site.

[From Benefits | Mollom]

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