Part of the reason not to let users pay is it could decrease the value of the ads they do show. The 8% or so of people who would pay for twitter are probably also the more influential, heavy twitter users. Thus, the 92% of people getting ads might be the lower-value users. That means their current ad revenue might drop drastically. But they could still make a few hundred million dollars from that 8%...
Good point. But remember, the 8% is presumably using third-party apps and not seeing ads now anyways. They would be willing to pay to continue to use the third-party apps ad-free.
I still believe that Twitter will not go for a premium option especially nowadays that there are a lot of social media web developers that are willing to compete for the million of people using free social media services.
