Currently, it's so damn difficult to get to apply ducal taxes, though. Most dukes don't ask for them, and lords can easily change ducal allegiance with extreme ease if ever you ask for more than they care to part with. And it's usually impossible to justify as you get more gold than any of them, too.
Duchy taxes will still be useless. You would have to levy ruinous taxes on rurals to make up even a modest 10% tax on most cities. Unless you had a LARGE duchy and could spread it out over many smaller regions. Which the majority of duchies don't have. One duke willing to take the income loss himself and not tax his vassals in return for the extra region allegiance would throw the tax curve out the window, too.
