"Seize peripheral duchy" is designed as a workaround to the very real problem that, in the game as it stands, it's basically impossible for the Byzantines to lose land. They can't suffer independence or dissolution factions, their governors get easy CBs that let them expand rapidly, and the ability to exceed normal MAA limits means they are also always going to be one of the strongest powers.
So there needs to be some way for them to lose land, and in particular for the player to weaken them. That's what it's for, and that's why the devs should be very reluctant to nerf it. As it stands, the AI rarely uses it.