Unh, I was hoping to promote magnetic shielding over gravimetric - but I guess it wouldn't matter in either case. In one case, you have a good chance to latch on the ship, in the other, you might not latch on it but you're still applying extra gravitons on the ship - so, it'd work either way.
When a graviton beam would be used on a ship priming its anti-interdiction, there would also be the need to keep the lock on the ship that one would want to avoid escaping... so, it gives an option to avoid escape, though it'd involve active effort, close proximity and fancy flying.
However, seeing that ships must be able to tow one another, I'd say that having a graviton beam active doesn't hinder the superluminal of hyperspace fold systems of the vessel using it... else we'd never be able to tow ships from one system to another.