What the big companies are realising is this: When you get lots of small developers using your API, you have tens of thousands of people who are actively marketing your product through your API.

You don't need to build up any large marketing force anymore, just let the developers do for you. For example, facebook connect drives people to facebook, etc. The web is becoming an OS, and the big companies are trying to achieve developer lock-in by offering a lot of convenience APIs and platforms.

He who gets the developers will get the users.