It's not just about the helicopters.
It's about the people fighting, and the heavy weapon emplacements helping them compensate for magically-enhanced durability on the other side, and the cover they're hiding behind and, perhaps more vital than all the others, THE STAR SHELLS. If Onogoro is once again free to shoot those down, then Team Good is stuck trying to fight people they can't see, with far less heavy weapon support and rapidly-eroding cover.
Yes, Nanoha hasn't intercepted all the ground-to-ground rocket attacks, but "not all" is still better than "none". And really it can't be overstated how much of an advantage being able to see your enemy is. Those star shells are vitally important to our side.