Here are Tim O’Reily’s 6 truths of free APIs. I definitely agree with this one –

The provider of your API will find it easier to implement services on top of their API than you will. Therefore you have to add something of your own that’s difficult to replicate, something beyond a simple UI tweak or a feature like “search”, so that the business that provides the API doesn’t simply compete with you when you look like you’re succeeding.