If your web service only provides one, first-class API through which all access flows, then you’ve only got one point to secure, you’re likely to have feature parity across interfaces, and the risk of marginalizing one interface is dramatically decreased. [Well put Daniel!]
WordPress To Disable Remote Access: If this sounds like a pipe dream, it’s worth pointing out that one very popular web service is already employing this strategy, and it works brilliantly. Flickr, Yahoo’s incredibly popular photo sharing site, is built on the very same APIs it makes available to clients. This results in some truly incredible Flickr-enabled applications and web services. And you don’t see any sign of Flickr disabling access to their API, because there’s too much at stake.