I will try again. I was intentionally vague because all of us live in different circumstances with different conditions so we have different needs. Some of the things that influenced my decision.
1. I have a closed flock. The only way new chicken blood is brought in to diversify the genetics is through hatching eggs. I do not expose mine to other chickens through shows, chicken swaps, or introducing new chicks of any age. None of my immediate neighbors have chickens. Exposure is minimal.
2. I will not have any chicken in my flock that is over three years old. According to the specialist I spoke to, that greatly reduces the risk of certain diseases. I don't remember specifically which ones because he just said we don't need to talk about certain others if I did not let my chickens age that much. I do remember they were not Marek's or Cocci, but several other diseases.
3. I live in an area that has a lot of commercial chicken production, especially chickens for the broiler market. That means that the local regulatory officials that work in this county know their stuff when it comes to chicken diseases. When and if an outbreak of anything happens, they react quickly and know what they are looking at. I know many people will be horrified that commercial operations are around, thinking they pose a danger to my flock. In my opinion as far as spreading diseases goes, I'd prefer to have commercial chicken operations around with their biosecurity measures than other free ranging unvaccinated flocks like mine. The commercial operations are at a greater risk from my chickens than mine are from theirs.
4. The hatcheries only offer vaccination for Cocci or Marek's. There are a lot of other diseases and vaccinations available for things that are worse than Cocci or Marek's.
5. There was only one reported case of Marek's in the last two years in my county and it was a pretty good distance away. That was in a small backyard flock like mine, not a commercial operation. It was quite a distance away. I'm sure there were other unreported cases of Marek's, but it was not running rampant in my area. From certain posts on this forum, there are some areas that Marek's is a serious threat. The vaccination for Marek's is turkey Marek's. The vaccination does not prevent Marek's, it prevents the lesions that cause the problems. It is quite possible that a chicken that has been vaccinated for Marek's has chicken Marek's and can give chicken Marek's to other chickens. I hatch a few batches of chicks every year, some under broodies and some in the incubator. Since you have to vaccinate chicks with the turkey Marek's vaccine before they are exposed to other chickens that have Marek's, that was not practical for me with the chicks that hatch under broodies. I also have my brooder in the coop, so any chicks that hatch in an incubator are immediately exposed to the other adult chickens. I saw little value in getting mine vaccinated for Marek's. Others find great value in it.
6. Cocci is caused by a protozoa that the chicks get from the ground. They can develop immunity to specific varieties of cocci protozoa by being exposed to it at a very young age and going through a cycle of constant reinfestation for a few weeks. There are several different strains of the cocci protozoa. Immunity to one does not give immunity to the others. The cocci vaccine does not handle all the strains of protozoa, or at least it did not when I researched it. That has been a while, I admit. There are some strains of protozoa that cause a real bad case of cocci, but most can be handled by keeping your brooder fairly dry. The problem is not that there is cocci present; the problem is that it gets too populous in their intestines. Cocci will breed in wet manure as well as in their intestines. They eat their own manure. If they eat enough cocci protozoa to add to what is in their intestines just breeding, the concentration can get high enough to seriously injure or kill them. I'm not worried about cocci and my broody raised chicks. They are exposed to cocci from day 1 and develop the immunity they need. I raise my brooder raised chicks on wire, so the brooder is very dry. I take some dirt from the run and feed it to the chicks so they are exposed to cocci protozoa from a very early age. Then I put a piece of plywood with a slightly raised edge around it in my brooder to put their first feed on and also to catch and hold some of their poop. It needs to be slightly damp so the protozoa can live in it but not wet where the protozoa population gets out of control. After a few weeks the chicks have developed immunity to the strains of cocci that I have in my run. So instead of vaccinating for some strains of cocci, I introduce it at an age they can develop immunity to the strains I have and manage it by keeping the brooder pretty dry.
The way you manage your chickens and your local circumstances are probably different than mine. I suggest you try finding out what your local conditions are and making your plans accordingly. What works in Cleveland, Ohio might not work in Boulder, Colorado or Perth, Australia.
Good luck! Hope this helps.