You put all your basic turrets under that forcefield blob to increase their longevity. Basic turrets and missile frigates have about the same range IIRC, so even though you're doing 1/4 damage, your basic turrets also have x3 bonus against artillery. Additionally, it will take the frigates forever to bust through the forcefields - if the basic turrets aren't protected, they'll get flattened in no time by the frigates. If the turrets are under the forcefields, and there are engineers to repair the forcefields the basic turrets will have significantly more time to pound on the frigates without being wiped, and with the Black Widow golem there to wreck engines, the Frigates won't be able to flee.
Mark 1 forcefields have what, 6 million hitpoints? Assuming mk2 frigates do 800 damage with a 12 second reload (I can't remember what the new numbers are for frigates, but is that a good approximation?) They're doing ~66 damage per second. That will take forever to get through one, let alone 5 forcefields, BUT with 5000 frigates on-planet you will lose whatever unshielded defenses you have deployed.
Usually you would not want to deploy your turrets under a forcefield, but in this case, where you have no chokepoint to spam with grav turrets and tractor beams. You don't even know where the ships are coming from in the system.