The challenge is that people really really don't want to see game invites / gifts / in-app purchases in their friends streams, unless they're personally playing the game, too (and even then they don't always like it).

Imagine you follow @timoreilly for his technical and political insights, only to see farmville spam in 3 out of 4 of his posts. That would get very old, very quickly.

Twitter is awesome partly because of the lack of app-specific streams, but it necessarily limits the types of things you can do on the platform. Now, they may reverse course on that direction as a consequence of pushing Cards so hard, but they're not there yet, and arguably will steadfastly continue to avoid going there.