I think how you quarantine is important. Just bringing birds in from a stable flock and keeping them separate is pretty worthless in my opinion. You can treat them for mites, lice and worms if you want, but those are more inconvenient things, not life threatening. What you are really worried about is the diseases that will wipe out your flock. If they are coming from a situation where they have been exposed to other birds, then it is different and a quarantine is probably a pretty good idea. But if they have been in a stable situation with no exposure to other chickens, then the flock can have a disease that it has developed an immunity to but can infect other chickens. That will not show up in a quarantine.
It is possible that either flock has a disease that will infect the other. Either flock, not just the incoming flock. I'll use cocci as an easy example but there are others. To me, the effective, efficient way to quarantine is to take a bird or two from each flock and put them together in isolation. You only put one or two of your birds at risk instead of your entire flock and you get a much better idea disease-wise of what will happen when you combine the two flocks.
I think quarantine is a very valuable tool but if it is not used correctly, it is not very useful.
It is possible that either flock has a disease that will infect the other. Either flock, not just the incoming flock. I'll use cocci as an easy example but there are others. To me, the effective, efficient way to quarantine is to take a bird or two from each flock and put them together in isolation. You only put one or two of your birds at risk instead of your entire flock and you get a much better idea disease-wise of what will happen when you combine the two flocks.
I think quarantine is a very valuable tool but if it is not used correctly, it is not very useful.