In my limited but growing experience dealing with free and paid users, I see it this way: when a customer pays for your product, it's like they're saying "I trust you." When you charge for something, you are asking for someone's trust because you believe in what you've created. You owe it to yourself to make people pay for what you build.

Maybe Aladdin said it best...do you trust me? (http://www.youtube.com/watch?v=kkwMEwenaQQ)