I love debating this question. I lose a lot of chickens to cocci. My first batch this spring, I lost over 25% of my meat birds.... and comparing notes with my neighbors, I did better than some people. I sell my meat birds marketed as no antiobiotic, so I already precluded myself from using the medicated stuff. However, on breeding animals which are not to be eaten or are not pregnant (goats and ewes), I use a coccidiostat. I've lost goats to Cocci trying to be "natural". It seemed like a great loss in hindsight that they died becasue I was being proud. Afterall, they weren't meant to be eaten.
Back to chickens. As the spring dries into summer, I lose far fewer birds to it. If you get meat birds in Spring, you'll lose more to it. If breeding your own meat birds, you would want to hold back the ones who seemed to grow best with the least issues. Selective breeding is really your only alternative to coccidiostats; at least the only sustainable alternative. I live in a wet area with wet soils. Cocci thrives here. No good management, clean litter/water, fresh grass daily will prevent it. I recall my extension agent laughing out loud when I told her "I though if I just kept good care of my meat birds, moved them daily, kept them healthy, I'd have no problem". I was frankly naive.
I currently have 50 freedom rangers out on grass at 4 weeks of age. I haven't lost a single one to disease yet. By this time, I bet 10 cornish X's would have dropped dead. The weather has been better, so it's a little like comparing apples to oranges. However, I think in the long run, I'll be staying away from the cornish X's.