Ad banners won't do much for them because the most dedicated Twitter users don't visit the Twitter site instead opting to use SMS, RSS, or some dedicated app to keep tabs on it. You could put them into the RSS for feed aggregators, but you couldn't send them through SMS and apps that do more than just blindly slurp the RSS are going to remove advertising. That pretty much leaves charging for some kind of service as their only option for making money. I think they might have something if they could convince the corporations that are essentially getting free advertising to pay for something, but I think even that is going to be an uphill battle.
And yes, I do use Twitter. Or at least, I have an account. I follow a few friends and coworkers and almost never post anything. I wouldn't keep using it if I had to pay for it.