Flak is pretty good for killing anything that doesn't have GVC, GVD, GTC, GTD, or GTVA in its name. If a group of hostile Elysiums or Azrael pass by anything with flak, its buh-bye transports. Freighters fall to the same fate (not mentioning the name *cough* POSEIDON *cough*), as well as science cruisers.
Like Dg said, without shields, flak is 1337. That is why, ladies and gentlemen, nobody makes a mission to attack an Aeolus (shiver) in subspace without Maxims or Trebs.