I would guess it would be for redundancy in case what ever powers the anti-grav generators failed. Since they looked like chemical thrusters, it does make you wonder why they weren't using them to slow their descent into the atmosphere?
In space manuvering to match up with the Vengance, dropping out of the sky...The Enterprise almost has a thruster on every open outside plane. I thought it was neat and it does help to explain the command "full stop" as I don't see a set of impulse engines on the front of the saucer.
As far as them not using them to slow their decent well, thats because everything was offline until Kirk kicked back into place the bottom part of the Death Star core. Apparently power for all types of thrust is channeled thru the warp core on the NuEnterprise.
The maneuvering thrusters are no use to decelerate a ship from half the speed of light to a dead stop. Instead even while travelling at subluminal speed, according to Memory Alpha driver coils, ...
from the impulse fusion reactors
to envelop the starship
in a low-energy subspace field
intended to lighten the relative mass
of the starship
it encompasses. This significantly reduces the mass burden on the impulse drive, allowing for unprecedented rates of acceleration.'
So in theory with the deactivation of the impulse coil and thus the dissolving of the 'subspace bubble', a starship returns to its former motion vector.
Well if what happens is a few fusion reactors creating a bubble which lightens the ship's mass causing it to have unprecedented rates of acceleration, what are the giant glowing red ports on the back of the saucer for? All that would be needed are thrusters if the ship's mass were reduced. That's what they did on DS9 to move it to the wormhole.
I don't know if increasing the mass of something in space causes deceleration per the theroy, maybe it does. Do suns slow their respective galactic speed as they go to red giants? I don't know what another example would be. But it is facinating to think that once the bubble is gone the ship just slows, that's pretty cool you nabbed that! I always wanted to know the how that was answered and why they didn't have another set of engines on the front.