I too find biosecurity to be silly...but if one is constantly moving birds with weak immune systems(heritage line show birds)in and out of the flock and exposing them to this or that group of birds from all points of the compass, I'm betting it's necessary.
What's not necessary is having birds with weak immune systems because one would rather crutch it up with meds than build from within, adding birds too often from various sources, keeping birds in small pens for extended periods of time, keeping birds in coop and run situations that compromise the soil structure in the runs, not culturing the bird's bowels with good cultures, not keeping good soil culture in their living environment, overstocking, not providing a more natural diet and habitat for fowl life.