In contrast, the old Yahoo API -- the Overture DTC-XML platform -- was closed. Access was/is limited to major agencies and advertisers. Not sure whether the new Yahoo platform will be open to the public. I suspect it wont. (Need to go back and read our agency T&Cs carefully to see.)
The open Google AdWords API led to wide adoption and the creation of many tools.
This pushed AdWords use out to more advertisers, making the AdWords market even more efficient (read: higher CPCs, more revenue to Google). Even though users are now grumbling about the coming Adwords API fees, the open API approach was a Very Smart Idea for Google.
But I have mixed feelings about the recent AdSense API announcement / leak...
If this new API provides good clicks to advertisers and provides crucial revenue to user generated content sites like YouTube and Facebook, well that is all well and good.
But my fear the API will allow AdSense spammers to do more damage more quickly. If made public, the automation enabled by an API could lower the quality of content traffic even more so, and lower it even more quickly. That'd turn out be a Very Dumb Idea for Google.
Google, don't open up the AdSense API to the world. Restrict it to major solid user-gen content sites. Don't let the spammers use it to automate their damage. The search community will thank you.