If you keep poultry you have your own 'set' of bacteria and viruses and your birds usually build up an immunity to the 'set' you have.
Some birds are carriers and look to be perfectly healthy and when you introduce them to other poultry, even at shows, you transfer one 'set' to another bird.
Buying from a local reliable breeder is no guarantee that the birds you buy are totally free of fleas,lice,mites, scaly leg or many of the other disease problems poultry carry. You take a huge risk of cross contamination every time you bring in a new bird/birds.
I run a closed flock here and never ever buy in birds, eggs yes, but I wouldn't want the risk of bringing in any viruses that my 400 birds cannot deal with.
Besides the problem of pecking order and new birds will be attacked ( Purple spray does not have any effect on the stress levels or bullying) your introducing strangers into a community that will defend the space they have and can result in serious damage to a bird.
Adding birds that have been vaccinated to birds that haven't introduces carriers of whatever the birds were vaccinated for. The untreated birds have no immunity to it.
I can do without your smart arse comments Plums. I think I know slightly more about poultry than you do my girl.
Perhaps I should clarify, I have a 'broody coop', in my case a house that takes 3-4 hybrids. This is where new birds go initially. it has its own enclosed run.
WHen I actually add them I follow the method I described above.
Lovely to hear from you Castle Farm, your comments are always so helpful