I had an MG scare once. All my birds started to sneeze a bit, one had a rattle in her breath, another had swelling around her eyes. I considered culling the whole flock after researching, thinking it was MG... but the more I researched, the more I found that it's near impossible to avoid MG and a LARGE percentage of flocks have it, without ever knowing.. as stress brings it to light.
I then decided I would not worry about it. Not at that time. I wouldn't cull until my birds were suffering.. which they were not.
A few weeks later everyone was back to normal, I had only given them some vet Rx and dewormed them (in case of gape worm). They had gotten wet in the rain before getting sick, and either got a cold (or stressed). They've not been sick since, and that was the winter of 2013-2014.
No one enters my coop but I (and rarely my kids/husband) and I don't bring adult birds in. Chicks are automatically quarantined, because they inhabit a brooder for their first weeks. I don't visit chicken swaps or auctions (those seem just the worst idea ever...). But, I do not believe wiping out an entire flock because they are carriers is the answer. Ofcourse, if your birds are deathly ill, and you need to end their suffering, that's a different story. But don't be too scared of MG, it's out there, it's common and birds can carry it without you ever knowing. No quarantine can garantee you wont introduce it to your flock, it can go undetected for years. Same with Marek's. Unless your birds are never going to see daylight, you can not protect them 100%. Wild birds fly over and poop in your run.
Biosecurity is important, but you simply can't avoid ALL risk. Culling your flock every time is costly, heart breaking and frankly unneccesary.