This ship is built to counter Klingon aggression. The Klingons have Bird-of-Preys that are fast, maneuverable ships. Smaller ships can overwhelm a larger ship.
That's why fleets, Klingon or otherwise, don't have just one type of ship but various ships in size and purpose. To work together, cover each others ass in all sorts of senarios.
You build Vengeance to counter the various huge Narada types that make quick work of your smaller ships, like we saw in ST09. And you build smaller ships to counter the various more maneuverable Bird of Prey types.
Vengeance is just another arrow in the quiver.