Or you try to ride the coattails of network effects and remove any obstacles to your information, index public data, and generally tries to weave in the service in the public web as tight as possible. This is the www model, as exemplified by Google.
Then there are the ones in between, such as newspapers, who generally wants to hide their information behind their service to increase the perceived value, but also wants to be reachable from the public web because without findability they have no value. This is of course paradoxical reasoning with no end of problems and no successful businesses.
Twitter from the start was part of the web 2.0 movement with a clear www strategy. Now they are doing a strategic shift. But how valuable is twitter when they're no longer indexed by services such as Google?
I can not see how they can grow their business in the long term from this. They lose the sentiment of a mirror of public discourse and instead will become like any web forum. So there will be zero journalistic value, and why would celebrities want to hang around then?
It's not even a slippery slope. It's just a slope.