I'm not opposed to it, but I imagine you would have a lot of people very hesitant to put their opinions on record with a system that is controlled by the government. The big benefit with Twitter is it's co-mingled with citizen and government employees alike and I'm not sure how you'd promote a government run system. Governments needs a subpoena to access records from a private company (theoretically) but if that layer of protection isn't there because the system is owned by a government, then I doubt anyone would want to use it for more than press releases, wedding announcements, births, obituaries and things like that.