"However if there are enough of these kinds of requests, it may point to an oppostunity [sic] for the website to provide an API. This often ends up building an ecosystem of services and applications aroung the website which ends up benefitting everyone. On the flip side, I often wonder about the balance between making it easy for users to get their data and making it easy for competitors to poach our users."
Dead right on the ecosystem. Sending C&Ds is what you'd expect from old media like Newscorp, who own MySpaces - the walled garden thing is like Compuserve all over again. Amazon and Ebay seem to have the balance right, then again they have both sustainable business models based on transaction volumes, not solely based on stickiness.
I think ultimately if your data is valuable to you, and you are suspcious of web2.0 business models, you'll eventually run your own services, just the way many of us run our own weblogs, mailing lists, email accounts, sites, chat servers and source repositories. I'm surprised Goog/MSFT/YHOO don't just give away source code for these kinds of services to run on ISP hosts - they can index data from anywhere, especially if that code contains a plugin to upload indexes to search engines. Maybe you don't actually need to host the services to to cross sell to people.