I’ve been playing around with Thinkup, which you should too, if you value any content you create for Twitter or Facebook. For the twitter side they’ve got this nice graph of clients you’ve used. Here is mine:
This, as far as I’m concerned, explains why you need an API. Less than 10% of the content that I’ve created in twitter comes from their official web interface. And even my most used interface only represents 1/2 of my content. This is only possible because Twitter has been API strong since the beginning.
If you are building a platform to publish information, it has to have a web services API. Otherwise you are going to massively limit the ways that your users can and will publish information.