ok thx i see the problem now flaks are really not that impressive vs shield fighters, and i tend to use smaller ships so that if they do get behind my ships i am in trouble, but bigger ships has less problem with that.
Basically -- think of fighters less as missiles and more as very weak and nimble frigates. You wouldn't rely on two vulcans to kill a Hound or three PD lasers to kill a Kite, so don't expect one of them to kill a fighter. Frag damage type is useful for things without shields and armor, so missiles, some of the weakest fighters without shields, and in special use cases, larger enemy ships after armor is stripped (locust is actually a decent finisher for this reason).
Non fragmentation PD is generally more expensive flux wise and OP wise when comparing on raw dps, though, so it can be useful to have a mix . Though generally I think everyone agrees that the burst PD laser is the best all around due to balanced damage type, high armor pen, reasonable flux ratio, accuracy, and range, even with a high-ish OP cost...