I don't know if Twitter has ever spoken about it but the lack of premium accounts has long puzzled me. Surely there are worthwhile features they could offer without degrading the free tier experience. My circles are perhaps atypical but for those of us who use it as a fairly important professional tool, a $50/year pro account would be a complete no-brainer. Maybe there aren't enough people like this to deal with the complications. It seems unlikely they haven't thought about it. But the lack is still surprising.