I think it was a trade off between being realistic and protecting Earth. Yes, they could send the Bastion after the Lucifer, but against the beam cannon on the Lucifer, it would last something like 3 shots. The difference between Destroyers in FS1 was far far greater than in FS2.
The fact is that, if the attack DID fail, they needed something in place to stop the Lucifer getting back out again, there's only one way in and out of the Sol system via subspace, so the only way out for the Lucifer was back along the route it had come. This meant another journey without shields, whereby the Bastion would have another shot at taking it down.