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.
Why do you think Twitter won't even make the option available for users to pay for the service?