The problem I can see is places in the world where there is no legal way to pay for it, where there's no PayPal, where most people don't have credit cards. Righ

The problem I can see is places in the world where there is no legal way to pay for it, where there's no PayPal, where most people don't have credit cards. Right now there's no barrier to joining Twitter - making it paid would create a new barrier. Having said that, I'm sure it would cut down on the number of spambots present, and so might have one or two advantages for those that could afford it.