I've got 2 vulcans on my enforcers instead of flak because they are so effective. I've seen them shoot down reapers, full volleys of harpoons, etc. They are certainly worse knife fighting weapons than lmg's, and the reduced range means they can't protect other ships, but they are fantastic point defense for the mounting ship. I'd say single lmg is... 20% as effective at PD, and double lmg maybe 30% as effective.
It seem i've got a personal grudge against frontward ballistic pd (and SO builds in general), the combination of short range and fast approach make taking down incoming fast volleys before those hits the shield feel somewhat unreliable.
Many players far more experienced than myself support extensive pd arrays, turns out i'm a vulture and my ships are lucky to even get 2 lmg/pd(beam).
Instead, a railgun/needler is top-tier kinetic, the enemy gets too scared about flux and only release dangerous missiles when panick-retreating, plus leaving larger slots for efficient explosive damage.
Shields can generally tank any non-sabot missile, fighters are a bigger issue because of their severe drain on flux and it happens that the aforementioned weaponry is quite good at thinning-out approaching squadrons.
Yes it is extremely vulnerable if isolated or flanked, but it's the fleet's objective to not end-up in this situation and keep attacking, frag dmg does very little regarding neutering the enemy. Dual-flak gets a pass on capitals due to obscene range and saturation potential.