I think that's tricky. If Twitter is perceived to be charging users to not be abused, that's obviously crappy. Paid accounts or not I think there should be better first-party options for controlling who you see and who sees you.

At the same time, if the APIs are fully open, third party clients would be able to build more powerful filtering features. In effect that would allow anyone to build entire sub communities on top of Twitter. This is starting to sound more and more like app.net...