It could be used as a way of attacking Bitcoin by congesting the network but it would only be a temporary attack as no one is going to send that much Bitcoin. How do I know this? You can attack Bitcoin the same way now by sending 0.0000001 transactions on the network with no fees and it would congest the network but no one does it because they cannot keep it up and it would only be a temporary attack. Lightening makes it easier to do with fees but it would not make much of a difference because of the congestion it causes does not achieve any thing.