I went to a mobile gaming summit once, and amusingly, companies were able to point out Facebook's moves away from this open season approach in their download number graphs. Facebook limits you to one picture per post? Big drop. Facebook hides game posts from people not playing the game? Big drop. Etc.. So I imagine if Facebook was forced to eventually start limiting game posts, Twitter will eventually have to follow, as you say.
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.