Debates will affect which endings and scene variations you get (which I won't spoil), but here are some hints for the debates currently in the game:Actually, what is the effect of winning debates with the NPC?
Dante
If you win, he'll be more optimistic and make relatively merciful choices as princeps. This will affect not only his own ending, but also the endings of some other characters.
Dredge
If you win, he'll let his hostage live, remove himself from the Empire, and give you maps of his under-city smuggling tunnels. The latter two will affect how the war turns out.
But in general most of the effects will be felt at the end. And keep in mind there's nothing wrong with losing a debate.