It's a good idea for flickr anyway (Score 5, Insightful)

As well as all the usual 'everything should be open arguments' there's a really simple reason why Flickr is right to implement this for their own reasons. It gives them great metrics on where their users are going. If Zoomr uses its api key to request a certain Flickr user's photos, and then that user becomes less active on Flickr, Flickr knows where the user has gone. This way it can see good data on its competitors and take any action necessary by producing features which specifically target one particular competitor.