I hope the FF guys will work with you to ensure that there will be only one Tornado in the future and not two independent forks.
It always hurts to throw away code that you put your blood and sweat in (in this case their custom network layer). But the advantages of rebasing this onto twisted are overwhelming for both sides.
There's a cost to adding a dependency on Twisted. It's one more thing to download. There's more code to understand when something goes wrong. And so on.
What are the benefits to me as a web programmer? If I understand correctly, my application will work as it did before. If that's true, then Twisted did not add any win for me.
The benefits you get are seamless integration with all the other twisted tools and a solid foundation under the relatively new Tornado stack.
And a bug in twisted will raise many more eyebrows than a bug in Tornado - hence they are more likely to be discovered by someone else before you.
There's more code to understand
I criticize twisted for usability and documentation problems. But as the underpinning for something else (with hopefully better usability) it seems perfectly adequate.